diff --git a/app/contracts/work_packages/base_contract.rb b/app/contracts/work_packages/base_contract.rb
index 2cb0ebb6bbdd..917cbece33cf 100644
--- a/app/contracts/work_packages/base_contract.rb
+++ b/app/contracts/work_packages/base_contract.rb
@@ -54,6 +54,9 @@ class BaseContract < ::ModelContract
               permission: :assign_versions do
       validate_target_versions_are_assignable
     end
+    # Observed versions have no deprecated single-value counterpart to coexist
+    # with, so unlike target versions they are always offered and always
+    # multi-valued.
     attribute :observed_in_versions,
               permission: :assign_versions do
       validate_observed_in_versions_are_assignable
diff --git a/app/models/activities/fetcher.rb b/app/models/activities/fetcher.rb
index f31e07c4c6d4..a50dfde3dc5f 100644
--- a/app/models/activities/fetcher.rb
+++ b/app/models/activities/fetcher.rb
@@ -124,7 +124,8 @@ def journals_of_event_set(events)
       journal_ids = events.map(&:event_id)
 
       Journal
-        .includes(:data, :customizable_journals, :attachable_journals, :target_version_journals, :bcf_comment)
+        .includes(:data, :customizable_journals, :attachable_journals, :work_package_version_journals,
+                  :bcf_comment)
         .find(journal_ids)
         .then { |journals| ::API::V3::Activities::ActivityEagerLoadingWrapper.wrap(journals) }
         .index_by(&:id)
diff --git a/app/models/journal.rb b/app/models/journal.rb
index ff0b3475e5bb..84b9bae66e67 100644
--- a/app/models/journal.rb
+++ b/app/models/journal.rb
@@ -60,6 +60,7 @@ class Journal < ApplicationRecord
   register_journal_formatter OpenProject::JournalFormatter::MeetingStartTime
   register_journal_formatter OpenProject::JournalFormatter::MeetingState
   register_journal_formatter OpenProject::JournalFormatter::MeetingWorkPackageId
+  register_journal_formatter OpenProject::JournalFormatter::ObservedInVersions
   register_journal_formatter OpenProject::JournalFormatter::ParticipantChange
   register_journal_formatter OpenProject::JournalFormatter::ProjectPhaseActive
   register_journal_formatter OpenProject::JournalFormatter::ProjectPhaseDates
@@ -122,13 +123,6 @@ class Journal < ApplicationRecord
   has_many :project_phase_journals, class_name: "Journal::ProjectPhaseJournal", dependent: :delete_all
   has_many :storable_journals, class_name: "Journal::StorableJournal", dependent: :delete_all
   has_many :work_package_version_journals, class_name: "Journal::WorkPackageVersionJournal", dependent: :delete_all
-  # Row lifecycle is owned by work_package_version_journals above.
-  # rubocop:disable Rails/HasManyOrHasOneDependent
-  has_many :target_version_journals,
-           -> { where(kind: "target") },
-           class_name: "Journal::WorkPackageVersionJournal",
-           inverse_of: :journal
-  # rubocop:enable Rails/HasManyOrHasOneDependent
 
   has_many :notifications, dependent: :destroy
 
@@ -146,6 +140,20 @@ class Journal < ApplicationRecord
 
   alias_attribute :internal, :restricted
 
+  # The snapshotted versions of a work package, split by the kind that
+  # references them.
+  #
+  # Deliberately derived in memory instead of being declared as kind-scoped
+  # associations: as associations, each kind would issue its own query, and
+  # every caller eager loads all of them together anyway.
+  def target_version_journals
+    work_package_version_journals.select { |journal| journal.kind == "target" }
+  end
+
+  def observed_in_version_journals
+    work_package_version_journals.select { |journal| journal.kind == "observed_in" }
+  end
+
   # In conjunction with the included Comparable module, allows comparison of journal records
   # based on their corresponding version numbers, creation timestamps and IDs.
   def <=>(other)
diff --git a/app/models/queries/work_packages.rb b/app/models/queries/work_packages.rb
index 26b3589eb3c8..1f648f49d273 100644
--- a/app/models/queries/work_packages.rb
+++ b/app/models/queries/work_packages.rb
@@ -59,6 +59,7 @@ module Queries::WorkPackages
     filter Filter::UpdatedAtFilter
     filter Filter::VersionFilter
     filter Filter::TargetVersionsFilter
+    filter Filter::ObservedInVersionsFilter
     filter Filter::WatcherFilter
     filter Filter::DatesIntervalFilter
     filter Filter::ParentFilter
diff --git a/app/models/queries/work_packages/filter/filter_on_target_versions_mixin.rb b/app/models/queries/work_packages/filter/filter_on_work_package_versions_mixin.rb
similarity index 78%
rename from app/models/queries/work_packages/filter/filter_on_target_versions_mixin.rb
rename to app/models/queries/work_packages/filter/filter_on_work_package_versions_mixin.rb
index 8a16f5e388c1..c9da6ec352bc 100644
--- a/app/models/queries/work_packages/filter/filter_on_target_versions_mixin.rb
+++ b/app/models/queries/work_packages/filter/filter_on_work_package_versions_mixin.rb
@@ -28,7 +28,12 @@
 # See COPYRIGHT and LICENSE files for more details.
 #++
 
-module Queries::WorkPackages::Filter::FilterOnTargetVersionsMixin
+# Filters work packages on the versions referenced through
+# work_package_versions.
+#
+# Including filters must define #version_kind, returning the
+# work_package_versions kind they match on.
+module Queries::WorkPackages::Filter::FilterOnWorkPackageVersionsMixin
   STATUS_BY_OPERATOR = { "o" => "open", "c" => "closed", "l" => "locked" }.freeze
 
   def allowed_values
@@ -57,7 +62,7 @@ def ar_object_filter?
   end
 
   def where
-    target_versions_where
+    versions_where
   end
 
   def value_objects
@@ -90,40 +95,40 @@ def versions
     end
   end
 
-  def target_versions_where
+  def versions_where
     case operator
     when "!" # is not
-      "NOT (#{target_version_matching_values})"
+      "NOT (#{version_matching_values})"
     when "!*" # empty
-      "NOT (#{any_target_version_associated})"
+      "NOT (#{any_version_associated})"
     when "*" # not empty
-      any_target_version_associated
+      any_version_associated
     when "o", "c", "l" # version status
-      target_version_with_status(STATUS_BY_OPERATOR[operator])
+      version_with_status(STATUS_BY_OPERATOR[operator])
     else # "=" is (or)
-      target_version_matching_values
+      version_matching_values
     end
   end
 
-  def any_target_version_associated
-    "EXISTS (#{target_associations.select(1).to_sql})"
+  def any_version_associated
+    "EXISTS (#{kind_associations.select(1).to_sql})"
   end
 
-  def target_version_matching_values
-    "EXISTS (#{target_associations.where(version_id: values).select(1).to_sql})"
+  def version_matching_values
+    "EXISTS (#{kind_associations.where(version_id: values).select(1).to_sql})"
   end
 
-  def target_version_with_status(status)
-    sub = target_associations
+  def version_with_status(status)
+    sub = kind_associations
             .joins(:version)
             .where(Version.table_name => { status: })
             .select(1)
     "EXISTS (#{sub.to_sql})"
   end
 
-  def target_associations
+  def kind_associations
     WorkPackageVersion
-      .where(kind: "target")
+      .where(kind: version_kind)
       .where("#{WorkPackageVersion.table_name}.work_package_id = #{WorkPackage.table_name}.id")
   end
 end
diff --git a/app/models/queries/work_packages/filter/observed_in_versions_filter.rb b/app/models/queries/work_packages/filter/observed_in_versions_filter.rb
new file mode 100644
index 000000000000..6200a3db3127
--- /dev/null
+++ b/app/models/queries/work_packages/filter/observed_in_versions_filter.rb
@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+class Queries::WorkPackages::Filter::ObservedInVersionsFilter <
+  Queries::WorkPackages::Filter::WorkPackageFilter
+  include ::Queries::WorkPackages::Filter::FilterOnWorkPackageVersionsMixin
+
+  def version_kind = "observed_in"
+
+  def self.key = :observed_in_version_id
+  def human_name = WorkPackage.human_attribute_name("observed_in_versions")
+end
diff --git a/app/models/queries/work_packages/filter/target_versions_filter.rb b/app/models/queries/work_packages/filter/target_versions_filter.rb
index 00ae4f35c814..806496dcd85f 100644
--- a/app/models/queries/work_packages/filter/target_versions_filter.rb
+++ b/app/models/queries/work_packages/filter/target_versions_filter.rb
@@ -30,7 +30,9 @@
 
 class Queries::WorkPackages::Filter::TargetVersionsFilter <
   Queries::WorkPackages::Filter::WorkPackageFilter
-  include ::Queries::WorkPackages::Filter::FilterOnTargetVersionsMixin
+  include ::Queries::WorkPackages::Filter::FilterOnWorkPackageVersionsMixin
+
+  def version_kind = "target"
 
   def self.key = :target_version_id
   def human_name = WorkPackage.human_attribute_name("target_versions")
diff --git a/app/models/queries/work_packages/filter/version_filter.rb b/app/models/queries/work_packages/filter/version_filter.rb
index 3593510bdc5e..f7d49d912202 100644
--- a/app/models/queries/work_packages/filter/version_filter.rb
+++ b/app/models/queries/work_packages/filter/version_filter.rb
@@ -32,7 +32,9 @@ class Queries::WorkPackages::Filter::VersionFilter <
   Queries::WorkPackages::Filter::WorkPackageFilter
   # Filters on `target_versions` as it is replacing
   # the legacy `work_packages.version_id` column.
-  include ::Queries::WorkPackages::Filter::FilterOnTargetVersionsMixin
+  include ::Queries::WorkPackages::Filter::FilterOnWorkPackageVersionsMixin
+
+  def version_kind = "target"
 
   def human_name
     WorkPackage.human_attribute_name("version")
diff --git a/app/models/queries/work_packages/selects/property_select.rb b/app/models/queries/work_packages/selects/property_select.rb
index 57c16655c41e..bd7f3ab1e625 100644
--- a/app/models/queries/work_packages/selects/property_select.rb
+++ b/app/models/queries/work_packages/selects/property_select.rb
@@ -144,6 +144,29 @@ def caption
             WHERE wpv.work_package_id = work_packages.id AND wpv.kind = 'target')
         SQL
     },
+    observed_in_versions: {
+      sortable: [
+        <<~SQL.squish,
+          (SELECT STRING_AGG(LOWER(v.name), ' ' ORDER BY LOWER(v.name), wpv.version_id)
+             FROM work_package_versions wpv
+             INNER JOIN versions v ON v.id = wpv.version_id
+            WHERE wpv.work_package_id = work_packages.id AND wpv.kind = 'observed_in')
+        SQL
+        <<~SQL.squish
+          (SELECT STRING_AGG(wpv.version_id::text, '.' ORDER BY LOWER(v.name), wpv.version_id)
+             FROM work_package_versions wpv
+             INNER JOIN versions v ON v.id = wpv.version_id
+            WHERE wpv.work_package_id = work_packages.id AND wpv.kind = 'observed_in')
+        SQL
+      ],
+      groupable:
+        <<~SQL.squish
+          (SELECT STRING_AGG(wpv.version_id::text, '.' ORDER BY LOWER(v.name), wpv.version_id)
+             FROM work_package_versions wpv
+             INNER JOIN versions v ON v.id = wpv.version_id
+            WHERE wpv.work_package_id = work_packages.id AND wpv.kind = 'observed_in')
+        SQL
+    },
     start_date: {
       sortable: "#{WorkPackage.table_name}.start_date"
     },
diff --git a/app/models/work_package/exports/formatters/observed_in_versions.rb b/app/models/work_package/exports/formatters/observed_in_versions.rb
new file mode 100644
index 000000000000..aa0879aa4ce0
--- /dev/null
+++ b/app/models/work_package/exports/formatters/observed_in_versions.rb
@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module WorkPackage::Exports
+  module Formatters
+    class ObservedInVersions < ::Exports::Formatters::Default
+      def self.apply?(attribute, _export_format)
+        attribute.to_sym == :observed_in_versions
+      end
+
+      def retrieve_value(object)
+        object.observed_in_versions.map(&:name)
+      end
+    end
+  end
+end
diff --git a/app/models/work_package/journalized.rb b/app/models/work_package/journalized.rb
index 9261067e9d25..d50d9d1f17fa 100644
--- a/app/models/work_package/journalized.rb
+++ b/app/models/work_package/journalized.rb
@@ -102,6 +102,7 @@ def self.event_url
     register_journal_formatted_fields /\Afile_links_?\d+\z/, formatter_key: :file_link
     register_journal_formatted_fields "project_phase_definition_id", formatter_key: :project_phase_definition
     register_journal_formatted_fields "target_versions", formatter_key: :target_versions
+    register_journal_formatted_fields "observed_in_versions", formatter_key: :observed_in_versions
 
     # Joined
     register_journal_formatted_fields :parent_id, :project_id,
diff --git a/app/models/work_package/versions.rb b/app/models/work_package/versions.rb
index 6c5df559c231..33980513ecfc 100644
--- a/app/models/work_package/versions.rb
+++ b/app/models/work_package/versions.rb
@@ -187,6 +187,18 @@ def effective_target_versions
     target_version_ids_replacements.filter_map { |id| versions_by_id[id] }
   end
 
+  # List of observed in versions, but takes into account pending overrides that
+  # were not written yet.
+  #
+  # There is no deprecated single-value column mirroring this kind, so unlike
+  # #effective_target_versions only the override has to be considered.
+  def effective_observed_in_versions
+    return observed_in_versions if observed_in_version_ids_replacements.nil?
+
+    versions_by_id = Version.where(id: observed_in_version_ids_replacements).index_by(&:id)
+    observed_in_version_ids_replacements.filter_map { |id| versions_by_id[id] }
+  end
+
   # An override can also originate from the system, e.g. when versions that are
   # not shared with the (new) project are cleared on a project change. Such
   # overrides are marked here so that contracts don't attribute them to the
diff --git a/app/services/journals/create_service/work_package_version.rb b/app/services/journals/create_service/work_package_version.rb
index d2474ba34ac7..db3eb0a55d06 100644
--- a/app/services/journals/create_service/work_package_version.rb
+++ b/app/services/journals/create_service/work_package_version.rb
@@ -30,11 +30,9 @@
 
 class Journals::CreateService
   # Journals the version associations of a work package. Only the kinds listed
-  # in JOURNALED_KINDS are snapshotted: observed_in versions stay unjournaled
-  # until the legacy version_id column stops being journaled, as they would
-  # otherwise render alongside it.
+  # in JOURNALED_KINDS are snapshotted.
   class WorkPackageVersion < Association
-    JOURNALED_KINDS = %w[target].freeze
+    JOURNALED_KINDS = %w[target observed_in].freeze
 
     def associated?
       journable.respond_to?(:target_versions)
diff --git a/app/services/work_packages/activities_tab/paginator.rb b/app/services/work_packages/activities_tab/paginator.rb
index f1176b8b78a8..f003ad2d3bee 100644
--- a/app/services/work_packages/activities_tab/paginator.rb
+++ b/app/services/work_packages/activities_tab/paginator.rb
@@ -178,8 +178,8 @@ def with_changesets(scope)
 
   def page_journals(page_relation)
     page_relation
-      .includes(:user, :customizable_journals, :attachable_journals, :storable_journals, :target_version_journals,
-                :notifications, :attachments)
+      .includes(:user, :customizable_journals, :attachable_journals, :storable_journals,
+                :work_package_version_journals, :notifications, :attachments)
       .to_a
   end
 
diff --git a/config/initializers/export_formats.rb b/config/initializers/export_formats.rb
index 793e3563e99d..e1015bce9c4e 100644
--- a/config/initializers/export_formats.rb
+++ b/config/initializers/export_formats.rb
@@ -52,6 +52,7 @@
       formatter WorkPackage, WorkPackage::Exports::Formatters::ProjectPhase
       formatter WorkPackage, WorkPackage::Exports::Formatters::SpentUnits
       formatter WorkPackage, WorkPackage::Exports::Formatters::TargetVersions
+      formatter WorkPackage, WorkPackage::Exports::Formatters::ObservedInVersions
 
       list Project, Projects::Exports::CSV
       list Project, Projects::Exports::PDF
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 84e0b983f81f..da1d422a670e 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -462,6 +462,7 @@ en:
           true: "include non-working days"
         journal_internal: Internal Journal
         notify: "Notify" # used in custom actions
+        observed_in_versions: "Observed versions"
         parent: "Parent"
         parent_issue: "Parent"
         parent_work_package: "Parent"
diff --git a/docs/api/apiv3/components/schemas/work_package_model.yml b/docs/api/apiv3/components/schemas/work_package_model.yml
index 648709496d3e..49d9e9f692cc 100644
--- a/docs/api/apiv3/components/schemas/work_package_model.yml
+++ b/docs/api/apiv3/components/schemas/work_package_model.yml
@@ -590,6 +590,19 @@ allOf:
               - Transitioning from single to multiple version support
               - (Temporary) Only allows a single value for compatibility with the version field
               - (Temporary) Must not be written together with `version` in the same request
+          observedInVersions:
+            type: array
+            items:
+              $ref: "./link.yml"
+            description: |-
+              List of versions the work package has been observed in
+
+              **Resource**: Collection of Version
+
+              # Conditions
+
+              - Unlike `targetVersions`, closed versions may be assigned
+              - Always multi-valued, independently of multiple version support
           watchers:
             allOf:
               - $ref: "./link.yml"
@@ -671,6 +684,9 @@ example:
     targetVersions:
       - href: "/api/v3/versions/1"
         title: Version 1
+    observedInVersions:
+      - href: "/api/v3/versions/2"
+        title: Version 2
     availableWatchers:
       href: "/api/v3/work_packages/1528/available_watchers"
     watch:
diff --git a/docs/api/apiv3/components/schemas/work_package_schema_model.yml b/docs/api/apiv3/components/schemas/work_package_schema_model.yml
index 97ae25c0f939..828110344355 100644
--- a/docs/api/apiv3/components/schemas/work_package_schema_model.yml
+++ b/docs/api/apiv3/components/schemas/work_package_schema_model.yml
@@ -90,6 +90,8 @@ properties:
     $ref: './schema_property_model.yml'
   targetVersions:
     $ref: './schema_property_model.yml'
+  observedInVersions:
+    $ref: './schema_property_model.yml'
   priority:
     $ref: './schema_property_model.yml'
   _links:
diff --git a/docs/api/apiv3/components/schemas/work_package_write_model.yml b/docs/api/apiv3/components/schemas/work_package_write_model.yml
index 483596c9689f..2efc036fb0a8 100644
--- a/docs/api/apiv3/components/schemas/work_package_write_model.yml
+++ b/docs/api/apiv3/components/schemas/work_package_write_model.yml
@@ -126,6 +126,19 @@ properties:
 
           - Transitioning from single to multiple version support
           - (Temporary) Only allows a single value for compatibility with the version field
+      observedInVersions:
+        type: array
+        items:
+          $ref: './link.yml'
+        description: |-
+          List of versions the work package has been observed in
+
+          **Resource**: Collection of Version
+
+          # Conditions
+
+          - Unlike `targetVersions`, closed versions may be assigned
+          - Always multi-valued, independently of multiple version support
       parent:
         allOf:
           - $ref: './link.yml'
diff --git a/docs/api/apiv3/tags/work_packages.yml b/docs/api/apiv3/tags/work_packages.yml
index bf37263b099b..3e29ebf95623 100644
--- a/docs/api/apiv3/tags/work_packages.yml
+++ b/docs/api/apiv3/tags/work_packages.yml
@@ -50,6 +50,7 @@ description: |-
   | type                   | The type of the work package                                                                                                                          | Type                   | not null                                  | READ / WRITE          |                                                                                             |
   | version                | The version associated to the work package                                                                                                            | Version                |                                           | READ / WRITE          |                                                                                             |
   | targetVersions         | List of versions associated to the work package                                                                                                       | []Version              |                                           | READ / WRITE          |                                                                                             |
+  | observedInVersions     | List of versions the work package has been observed in                                                                                                 | []Version              |                                           | READ / WRITE          |                                                                                             |
   | watchers               | All users that are currently watching this work package                                                                                               | Collection             |                                           | READ                  | **Permission** view work package watchers                                                   |
 
   ## Local Properties
diff --git a/frontend/src/app/features/work-packages/components/wp-edit/work-package-changeset.ts b/frontend/src/app/features/work-packages/components/wp-edit/work-package-changeset.ts
index dd948d7ad67f..18a893cbab52 100644
--- a/frontend/src/app/features/work-packages/components/wp-edit/work-package-changeset.ts
+++ b/frontend/src/app/features/work-packages/components/wp-edit/work-package-changeset.ts
@@ -62,13 +62,18 @@ export class WorkPackageChangeset extends ResourceChangeset<WorkPackageResource>
       delete (payload as { subject?:string }).subject;
     }
 
-    // Explicitly exclude the targetVersions if it's empty
+    // Explicitly exclude the version collections if they are empty
     if (isNewResource(this.pristineResource)) {
-      const links = (payload as { _links?:{ targetVersions?:unknown[] } })._links;
-      const targetVersions = links?.targetVersions;
+      const links = (payload as { _links?:Record<string, unknown> })._links;
 
-      if (links && (!Array.isArray(targetVersions) || targetVersions.length === 0)) {
-        delete links.targetVersions;
+      if (links) {
+        ['targetVersions', 'observedInVersions'].forEach((attribute) => {
+          const value = links[attribute];
+
+          if (!Array.isArray(value) || value.length === 0) {
+            delete links[attribute];
+          }
+        });
       }
     }
 
diff --git a/frontend/src/app/shared/components/fields/edit/edit-field.initializer.ts b/frontend/src/app/shared/components/fields/edit/edit-field.initializer.ts
index 01967f873396..8318d2b66339 100644
--- a/frontend/src/app/shared/components/fields/edit/edit-field.initializer.ts
+++ b/frontend/src/app/shared/components/fields/edit/edit-field.initializer.ts
@@ -143,7 +143,7 @@ export function initializeCoreEditFields(editFieldService:EditFieldService, sele
         'WorkPackage',
         VersionsEditFieldComponent,
         'versions',
-        ['targetVersions'],
+        ['targetVersions', 'observedInVersions'],
       )
       .addSpecificFieldType('Project', ProjectStatusEditFieldComponent, 'status', ['status'])
       .addSpecificFieldType('Portfolio', ProjectStatusEditFieldComponent, 'status', ['status'])
diff --git a/frontend/src/app/shared/components/fields/edit/field-types/versions-edit-field.component.ts b/frontend/src/app/shared/components/fields/edit/field-types/versions-edit-field.component.ts
index 22ca2a1ae82f..b1d6569c614b 100644
--- a/frontend/src/app/shared/components/fields/edit/field-types/versions-edit-field.component.ts
+++ b/frontend/src/app/shared/components/fields/edit/field-types/versions-edit-field.component.ts
@@ -39,13 +39,14 @@ import { CurrentProjectService } from 'core-app/core/current-project/current-pro
 import { HalResourceNotificationService } from 'core-app/features/hal/services/hal-resource-notification.service';
 
 /**
- * Edit field for the targetVersions attribute of work packages.
+ * Edit field for the version collection attributes of work packages
+ * (targetVersions, observedInVersions).
  *
- * The attribute always reads and writes a collection, but as long as the
- * multiple versions setting is inactive, the schema restricts it to a single
- * value (options.multiple). In that mode the field mimics the single select
- * fields it stands in for: an explicit "-" option, no save/cancel controls,
- * saving right on selection.
+ * The attributes always read and write a collection, but the schema may
+ * restrict one to a single value (options.multiple) — targetVersions does so as
+ * long as the multiple versions setting is inactive. In that mode the field
+ * mimics the single select fields it stands in for: an explicit "-" option, no
+ * save/cancel controls, saving right on selection.
  *
  * Versions can be created from within the field when the user is allowed to
  * (mirroring VersionAutocompleterComponent).
diff --git a/lib/api/v3/activities/activity_eager_loading_wrapper.rb b/lib/api/v3/activities/activity_eager_loading_wrapper.rb
index ad191acdbc3d..f3f96777d312 100644
--- a/lib/api/v3/activities/activity_eager_loading_wrapper.rb
+++ b/lib/api/v3/activities/activity_eager_loading_wrapper.rb
@@ -150,7 +150,8 @@ def predecessor_journals(journals)
                   ) AS journals
                 SQL
               )
-              .includes(:attachable_journals, :customizable_journals, :storable_journals, :target_version_journals)
+              .includes(:attachable_journals, :customizable_journals, :storable_journals,
+                        :work_package_version_journals)
           end
         end
       end
diff --git a/lib/api/v3/queries/schemas/observed_in_versions_filter_dependency_representer.rb b/lib/api/v3/queries/schemas/observed_in_versions_filter_dependency_representer.rb
new file mode 100644
index 000000000000..9935428d2c31
--- /dev/null
+++ b/lib/api/v3/queries/schemas/observed_in_versions_filter_dependency_representer.rb
@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module API
+  module V3
+    module Queries
+      module Schemas
+        # The allowed values of the observed versions filter are the same
+        # collection of versions the single version filter offers, so the
+        # dependency representation is identical. A dedicated class is still
+        # needed, as FilterDependencyRepresenterFactory resolves the representer
+        # from the filter's class name.
+        class ObservedInVersionsFilterDependencyRepresenter <
+          VersionFilterDependencyRepresenter
+        end
+      end
+    end
+  end
+end
diff --git a/lib/api/v3/work_packages/eager_loading/checksum.rb b/lib/api/v3/work_packages/eager_loading/checksum.rb
index c7bb0c0e707e..f94bed8601a2 100644
--- a/lib/api/v3/work_packages/eager_loading/checksum.rb
+++ b/lib/api/v3/work_packages/eager_loading/checksum.rb
@@ -54,19 +54,20 @@ def fetch_checksums_for(work_packages)
 
             protected
 
-            # The target versions associated via work_package_versions are a
-            # has_many, which the left_joins/pluck design above cannot express
-            # (it would multiply rows), so they enter the checksum as an
-            # aggregated correlated subquery. This also covers versions beyond
-            # the first, which the deprecated version association could not see.
-            # Only "target" rows are folded in: they are what the representer
-            # renders (as `version` and `targetVersions`); observed_in versions
-            # do not appear in the representation and must not bust its cache.
+            # The versions associated via work_package_versions are a has_many,
+            # which the left_joins/pluck design above cannot express (it would
+            # multiply rows), so they enter the checksum as an aggregated
+            # correlated subquery. This also covers versions beyond the first,
+            # which the deprecated version association could not see.
+            # Every kind is folded in, since the representer renders both the
+            # target (as `version` and `targetVersions`) and the observed_in
+            # rows (as `observedInVersions`). The kind is part of the aggregated
+            # value so that moving a version between kinds busts the cache too.
             VERSIONS_CHECKSUM_SQL = <<~SQL.squish
-              (SELECT COALESCE(STRING_AGG(CONCAT(v.id, v.updated_at), ',' ORDER BY v.id), '')
+              (SELECT COALESCE(STRING_AGG(CONCAT(wpv.kind, v.id, v.updated_at), ',' ORDER BY wpv.kind, v.id), '')
                  FROM work_package_versions wpv
                  INNER JOIN versions v ON v.id = wpv.version_id
-                WHERE wpv.work_package_id = work_packages.id AND wpv.kind = 'target')
+                WHERE wpv.work_package_id = work_packages.id)
             SQL
 
             def md5_concat
diff --git a/lib/api/v3/work_packages/schema/specific_work_package_schema.rb b/lib/api/v3/work_packages/schema/specific_work_package_schema.rb
index a62908b970a3..ad86dc0b534d 100644
--- a/lib/api/v3/work_packages/schema/specific_work_package_schema.rb
+++ b/lib/api/v3/work_packages/schema/specific_work_package_schema.rb
@@ -56,6 +56,7 @@ def initialize(work_package:)
                    :assignable_priorities,
                    :assignable_versions,
                    :assignable_target_versions,
+                   :assignable_observed_in_versions,
                    :assignable_budgets,
                    :assignable_project_phases,
                    to: :contract
diff --git a/lib/api/v3/work_packages/schema/work_package_schema_representer.rb b/lib/api/v3/work_packages/schema/work_package_schema_representer.rb
index 79c9d7427051..504dddec0cf9 100644
--- a/lib/api/v3/work_packages/schema/work_package_schema_representer.rb
+++ b/lib/api/v3/work_packages/schema/work_package_schema_representer.rb
@@ -334,6 +334,22 @@ def initialize(schema, self_link:, **context)
                                          required: false,
                                          options: -> { { multiple: Setting::WorkPackageMultipleVersions.active? } }
 
+          # Unlike target versions, observed versions have no single-valued
+          # predecessor to stand in for, so the attribute is always offered and
+          # always a collection.
+          schema_with_allowed_collection :observed_in_versions,
+                                         type: "[]Version",
+                                         value_representer: Versions::VersionRepresenter,
+                                         link_factory: ->(version) {
+                                           {
+                                             href: api_v3_paths.version(version.id),
+                                             title: version.name
+                                           }
+                                         },
+                                         writable: ->(*) { represented.writable?(:observed_in_versions) },
+                                         required: false,
+                                         options: -> { { multiple: true } }
+
           schema_with_allowed_collection :priority,
                                          value_representer: Priorities::PriorityRepresenter,
                                          link_factory: ->(priority) {
diff --git a/lib/api/v3/work_packages/work_package_representer.rb b/lib/api/v3/work_packages/work_package_representer.rb
index d7dac7f07d58..fcd237dfe1a0 100644
--- a/lib/api/v3/work_packages/work_package_representer.rb
+++ b/lib/api/v3/work_packages/work_package_representer.rb
@@ -606,6 +606,31 @@ def self_v3_path(*)
                                represented.target_version_ids = parse_link_ids_from_fragment(fragment, :version).compact
                              end
 
+        associated_resources :observed_in_versions,
+                             v3_path: :version,
+                             representer: ::API::V3::Versions::VersionRepresenter,
+                             getter: ->(*) {
+                               next unless embed_link?(:observed_in_versions)
+
+                               represented.effective_observed_in_versions.map do |version|
+                                 ::API::V3::Versions::VersionRepresenter.create(version, current_user:)
+                               end
+                             },
+                             link: ->(*) {
+                               represented.effective_observed_in_versions.map do |version|
+                                 ::API::Decorators::LinkObject
+                                   .new(version,
+                                        property_name: :itself,
+                                        path: :version,
+                                        getter: :id,
+                                        title_attribute: :name)
+                                   .to_hash
+                               end
+                             },
+                             setter: ->(fragment:, **) do
+                               represented.observed_in_version_ids = parse_link_ids_from_fragment(fragment, :version).compact
+                             end
+
         associated_resource :parent,
                             v3_path: :work_package,
                             representer: ::API::V3::WorkPackages::WorkPackageRepresenter,
@@ -830,7 +855,8 @@ def ordered_custom_actions
                                 watchers
                                 attachments
                                 budget
-                                target_versions]
+                                target_versions
+                                observed_in_versions]
 
         # The dynamic class generation introduced because of the custom fields interferes with
         # the class naming as well as prevents calls to super
diff --git a/lib/open_project/journal_formatter/joined_versions.rb b/lib/open_project/journal_formatter/joined_versions.rb
new file mode 100644
index 000000000000..e9a8e681019f
--- /dev/null
+++ b/lib/open_project/journal_formatter/joined_versions.rb
@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+# Renders the change to a set of versions referenced by a work package. Each
+# value is the sorted, comma-joined version ids (see JournalChanges); every id
+# is resolved to the version's name, dropping versions that have been deleted in
+# the meantime.
+class OpenProject::JournalFormatter::JoinedVersions < JournalFormatter::NamedAssociation
+  private
+
+  def format_values(values, key, cache:)
+    klass = class_from_field(key)
+
+    values.map do |value|
+      next if value.blank? || klass.nil?
+
+      value.to_s.split(",")
+           .filter_map { |id| associated_object(klass, id.to_i, cache:)&.name }
+           .join(", ")
+           .presence
+    end
+  end
+end
diff --git a/lib/open_project/journal_formatter/observed_in_versions.rb b/lib/open_project/journal_formatter/observed_in_versions.rb
new file mode 100644
index 000000000000..f99881cf4ae8
--- /dev/null
+++ b/lib/open_project/journal_formatter/observed_in_versions.rb
@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+# Renders the change to the set of versions a work package was observed in
+# (see JournalChanges#get_observed_in_versions_changes).
+class OpenProject::JournalFormatter::ObservedInVersions < OpenProject::JournalFormatter::JoinedVersions
+end
diff --git a/lib/open_project/journal_formatter/target_versions.rb b/lib/open_project/journal_formatter/target_versions.rb
index 81cb1676f91b..38a552689e6a 100644
--- a/lib/open_project/journal_formatter/target_versions.rb
+++ b/lib/open_project/journal_formatter/target_versions.rb
@@ -28,11 +28,9 @@
 # See COPYRIGHT and LICENSE files for more details.
 #++
 
-# Renders the change to the set of target versions. Each value is the sorted,
-# comma-joined version ids (see JournalChanges#get_target_versions_changes);
-# every id is resolved to the version's name, dropping versions that have
-# been deleted in the meantime.
-class OpenProject::JournalFormatter::TargetVersions < JournalFormatter::NamedAssociation
+# Renders the change to the set of target versions
+# (see JournalChanges#get_target_versions_changes).
+class OpenProject::JournalFormatter::TargetVersions < OpenProject::JournalFormatter::JoinedVersions
   private
 
   # While the multiple versions feature is inactive, the rest of the UI still
@@ -44,17 +42,4 @@ def label(key)
       super("version")
     end
   end
-
-  def format_values(values, key, cache:)
-    klass = class_from_field(key)
-
-    values.map do |value|
-      next if value.blank? || klass.nil?
-
-      value.to_s.split(",")
-           .filter_map { |id| associated_object(klass, id.to_i, cache:)&.name }
-           .join(", ")
-           .presence
-    end
-  end
 end
diff --git a/lib_static/plugins/acts_as_journalized/lib/journal_changes.rb b/lib_static/plugins/acts_as_journalized/lib/journal_changes.rb
index b933ca03c73e..4e4bf75925f2 100644
--- a/lib_static/plugins/acts_as_journalized/lib/journal_changes.rb
+++ b/lib_static/plugins/acts_as_journalized/lib/journal_changes.rb
@@ -33,20 +33,7 @@ def get_changes
     return @changes if @changes
     return {} if data.nil?
 
-    changes = [
-      get_cause_changes,
-      get_data_changes,
-      get_attachments_changes,
-      get_custom_comments_changes,
-      get_custom_fields_changes,
-      get_project_phases_changes,
-      get_target_versions_changes,
-      get_file_links_changes,
-      get_participants_changes,
-      get_agenda_items_changes
-    ].compact
-
-    merged = changes.reduce({}.with_indifferent_access, :merge!)
+    merged = all_changes.reduce({}.with_indifferent_access, :merge!)
 
     @changes = suppress_mirrored_version_change(merged)
   end
@@ -143,6 +130,18 @@ def get_target_versions_changes
     { target_versions: [old_value, new_value] }
   end
 
+  # Diffed as a single value just like the target versions above: one
+  # "Observed versions" line with the old and the new list.
+  def get_observed_in_versions_changes
+    return unless journable.respond_to?(:observed_in_versions)
+
+    old_value = predecessor && joined_observed_in_version_ids(predecessor)
+    new_value = joined_observed_in_version_ids(self)
+    return if old_value == new_value
+
+    { observed_in_versions: [old_value, new_value] }
+  end
+
   def get_file_links_changes
     return unless has_file_links?
 
@@ -187,6 +186,22 @@ def get_participants_changes
 
   private
 
+  def all_changes
+    [
+      get_cause_changes,
+      get_data_changes,
+      get_attachments_changes,
+      get_custom_comments_changes,
+      get_custom_fields_changes,
+      get_project_phases_changes,
+      get_target_versions_changes,
+      get_observed_in_versions_changes,
+      get_file_links_changes,
+      get_participants_changes,
+      get_agenda_items_changes
+    ].compact
+  end
+
   # While the deprecated version_id column mirrors the target versions, a
   # version change diffs under both keys; only the target_versions
   # representation is rendered. Historical journals render the same way,
@@ -201,6 +216,10 @@ def joined_target_version_ids(journal)
     journal.target_version_journals.map(&:version_id).sort.join(",").presence
   end
 
+  def joined_observed_in_version_ids(journal)
+    journal.observed_in_version_journals.map(&:version_id).sort.join(",").presence
+  end
+
   def participant_baseline_journal
     journals = journable.journals.to_a
     current_index = journals.index { |entry| entry.id == id }
diff --git a/modules/xls_export/app/models/xls_export/work_package/exporter/xls.rb b/modules/xls_export/app/models/xls_export/work_package/exporter/xls.rb
index 5d40e69bf447..0a996853a0cb 100644
--- a/modules/xls_export/app/models/xls_export/work_package/exporter/xls.rb
+++ b/modules/xls_export/app/models/xls_export/work_package/exporter/xls.rb
@@ -35,7 +35,7 @@ class XLS < WorkPackage::Exports::QueryExporter
 
     def records
       work_packages
-        .includes(:assigned_to, :type, :priority, :category, :version, :target_versions)
+        .includes(:assigned_to, :type, :priority, :category, :version, :target_versions, :observed_in_versions)
     end
 
     def spreadsheet_title
diff --git a/spec/lib/api/v3/queries/schemas/observed_in_versions_filter_dependency_representer_spec.rb b/spec/lib/api/v3/queries/schemas/observed_in_versions_filter_dependency_representer_spec.rb
new file mode 100644
index 000000000000..ada73f80c039
--- /dev/null
+++ b/spec/lib/api/v3/queries/schemas/observed_in_versions_filter_dependency_representer_spec.rb
@@ -0,0 +1,165 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+require "spec_helper"
+
+RSpec.describe API::V3::Queries::Schemas::ObservedInVersionsFilterDependencyRepresenter do
+  include API::V3::Utilities::PathHelper
+
+  let(:project) { build_stubbed(:project) }
+  let(:query) { build_stubbed(:query, project:) }
+  let(:filter) { Queries::WorkPackages::Filter::ObservedInVersionsFilter.create!(context: query) }
+  let(:form_embedded) { false }
+
+  let(:instance) do
+    described_class.new(filter,
+                        operator,
+                        form_embedded:)
+  end
+
+  subject(:generated) { instance.to_json }
+
+  describe "generation" do
+    describe "properties" do
+      describe "values" do
+        let(:path) { "values" }
+        let(:type) { "[]Version" }
+        let(:order) { "sortBy=#{CGI.escape(JSON.dump([%i(name asc)]))}&pageSize=-1" }
+
+        context "for operator 'Queries::Operators::All'" do
+          let(:operator) { Queries::Operators::All }
+
+          it_behaves_like "filter dependency empty"
+        end
+
+        context "for operator 'Queries::Operators::None'" do
+          let(:operator) { Queries::Operators::None }
+
+          it_behaves_like "filter dependency empty"
+        end
+
+        context "within project" do
+          let(:href) do
+            "#{api_v3_paths.versions_by_workspace(project.id)}?#{order}"
+          end
+
+          context "for operator 'Queries::Operators::Equals'" do
+            let(:operator) { Queries::Operators::Equals }
+
+            it_behaves_like "filter dependency with allowed link"
+          end
+
+          context "for operator 'Queries::Operators::NotEquals'" do
+            let(:operator) { Queries::Operators::NotEquals }
+
+            it_behaves_like "filter dependency with allowed link"
+          end
+        end
+
+        context "without a project" do
+          let(:project) { nil }
+          let(:href) do
+            "#{api_v3_paths.versions}?#{order}"
+          end
+
+          context "for operator 'Queries::Operators::Equals'" do
+            let(:operator) { Queries::Operators::Equals }
+
+            it_behaves_like "filter dependency with allowed link"
+          end
+
+          context "for operator 'Queries::Operators::NotEquals'" do
+            let(:operator) { Queries::Operators::NotEquals }
+
+            it_behaves_like "filter dependency with allowed link"
+          end
+        end
+      end
+    end
+
+    describe "caching" do
+      let(:operator) { Queries::Operators::Equals }
+      let(:other_project) { build_stubbed(:project) }
+
+      before do
+        allow(instance).to receive(:to_hash).and_call_original
+
+        # fill the cache
+        instance.to_json
+      end
+
+      it "is cached" do
+        instance.to_json
+
+        expect(instance)
+          .to have_received(:to_hash).once
+      end
+
+      it "busts the cache on a different operator" do
+        instance.send(:operator=, Queries::Operators::NotEquals)
+
+        instance.to_json
+
+        expect(instance)
+          .to have_received(:to_hash).twice
+      end
+
+      it "busts the cache on a different project" do
+        query.project = other_project
+
+        instance.to_json
+
+        expect(instance)
+          .to have_received(:to_hash).twice
+      end
+
+      it "busts the cache on changes to the locale" do
+        I18n.with_locale(:de) do
+          instance.to_json
+        end
+
+        expect(instance)
+          .to have_received(:to_hash).twice
+      end
+
+      it "busts the cache on different form_embedded" do
+        embedded_instance = described_class.new(filter,
+                                                operator,
+                                                form_embedded: !form_embedded)
+        allow(embedded_instance).to receive(:to_hash).and_call_original
+
+        embedded_instance.to_json
+
+        expect(embedded_instance)
+          .to have_received(:to_hash).once
+      end
+    end
+  end
+end
diff --git a/spec/lib/api/v3/work_packages/eager_loading/cache_checksum_integration_spec.rb b/spec/lib/api/v3/work_packages/eager_loading/cache_checksum_integration_spec.rb
index cab3e50536db..43ac9482935d 100644
--- a/spec/lib/api/v3/work_packages/eager_loading/cache_checksum_integration_spec.rb
+++ b/spec/lib/api/v3/work_packages/eager_loading/cache_checksum_integration_spec.rb
@@ -145,12 +145,12 @@
         .not_to eql orig_checksum
     end
 
-    it "produces the same checksum on changes to an observed_in version" do
+    it "produces a different checksum on changes to an observed_in version" do
       other_version = create(:version, project:)
       work_package.work_package_versions.create!(version: other_version, kind: "observed_in")
 
       expect(new_checksum)
-        .to eql orig_checksum
+        .not_to eql orig_checksum
     end
 
     it "produces a different checksum on changes to the type id" do
diff --git a/spec/lib/open_project/journal_formatter/observed_in_versions_spec.rb b/spec/lib/open_project/journal_formatter/observed_in_versions_spec.rb
new file mode 100644
index 000000000000..52f0dcb4d06c
--- /dev/null
+++ b/spec/lib/open_project/journal_formatter/observed_in_versions_spec.rb
@@ -0,0 +1,108 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+require "spec_helper"
+
+RSpec.describe OpenProject::JournalFormatter::ObservedInVersions do
+  describe "#render" do
+    let(:version) { build_stubbed(:version, name: "Alpha") }
+    let(:other_version) { build_stubbed(:version, name: "Beta") }
+    let(:work_package) { build_stubbed(:work_package) }
+    let(:journal) { build_stubbed(:work_package_journal, journable: work_package) }
+    let(:instance) { described_class.new(journal) }
+    # Unlike the target versions formatter, the label never falls back to the
+    # single-version wording, the attribute having no such counterpart.
+    let(:label) { "<strong>Observed versions</strong>" }
+
+    before do
+      allow(Version).to receive(:find_by).and_return(nil)
+
+      [version, other_version].each do |v|
+        allow(Version).to receive(:find_by).with(id: v.id).and_return(v)
+      end
+    end
+
+    context "when setting observed versions" do
+      it "renders the version names as the new value" do
+        expect(instance.render(:observed_in_versions, [nil, "#{version.id},#{other_version.id}"]))
+          .to eq(I18n.t(:text_journal_set_to, label:, value: "<i>Alpha, Beta</i>"))
+      end
+    end
+
+    context "when changing observed versions" do
+      it "renders the old and new version names" do
+        expect(instance.render(:observed_in_versions, [version.id.to_s, other_version.id.to_s]))
+          .to eq(I18n.t(:text_journal_changed_plain,
+                        label:,
+                        linebreak: nil,
+                        old: "<i>Alpha</i>",
+                        new: "<i>Beta</i>"))
+      end
+    end
+
+    context "when removing all observed versions" do
+      it "renders the old version names as deleted" do
+        expect(instance.render(:observed_in_versions, ["#{version.id},#{other_version.id}", nil]))
+          .to eq(I18n.t(:text_journal_deleted, label:, old: "<strike><i>Alpha, Beta</i></strike>"))
+      end
+    end
+
+    context "with a version that no longer exists" do
+      it "renders only the existing version names" do
+        expect(instance.render(:observed_in_versions, [nil, "#{version.id},99999"]))
+          .to eq(I18n.t(:text_journal_set_to, label:, value: "<i>Alpha</i>"))
+      end
+    end
+
+    context "when the multiple versions feature is active",
+            with_flag: { work_package_multiple_versions: true },
+            with_settings: { work_package_multiple_versions: true } do
+      it "keeps the same label" do
+        expect(instance.render(:observed_in_versions, [version.id.to_s, other_version.id.to_s]))
+          .to eq(I18n.t(:text_journal_changed_plain,
+                        label:,
+                        linebreak: nil,
+                        old: "<i>Alpha</i>",
+                        new: "<i>Beta</i>"))
+      end
+    end
+
+    context "with html: false" do
+      it "renders plain text" do
+        expect(instance.render(:observed_in_versions, [version.id.to_s, other_version.id.to_s], html: false))
+          .to eq(I18n.t(:text_journal_changed_plain,
+                        label: "Observed versions",
+                        linebreak: nil,
+                        old: "Alpha",
+                        new: "Beta"))
+      end
+    end
+  end
+end
diff --git a/spec/models/queries/work_packages/filter/observed_in_versions_filter_spec.rb b/spec/models/queries/work_packages/filter/observed_in_versions_filter_spec.rb
new file mode 100644
index 000000000000..c1193fe5c377
--- /dev/null
+++ b/spec/models/queries/work_packages/filter/observed_in_versions_filter_spec.rb
@@ -0,0 +1,281 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+require "spec_helper"
+
+RSpec.describe Queries::WorkPackages::Filter::ObservedInVersionsFilter do
+  let(:actual_project) { create(:project) }
+  let(:version) { create(:version, project: actual_project) }
+  let(:other_project_version) { create(:version, project: create(:project)) }
+
+  let(:role) { create(:project_role, permissions: %i[view_work_packages]) }
+  let(:user) { create(:user, member_with_roles: { actual_project => role }) }
+
+  before { login_as(user) }
+
+  it_behaves_like "basic query filter" do
+    let(:project) { actual_project }
+    let(:type) { :list_optional }
+    let(:class_key) { :observed_in_version_id }
+    let(:values) { [version.id.to_s] }
+    let(:name) { WorkPackage.human_attribute_name("observed_in_versions") }
+
+    # Unlike the target versions filter, this one has no deprecated
+    # single-valued counterpart to alternate with, so it is offered regardless
+    # of the multiple versions feature.
+    describe "#available?" do
+      context "with the feature flag and the setting enabled",
+              with_flag: { work_package_multiple_versions: true },
+              with_settings: { work_package_multiple_versions: true } do
+        it "is available" do
+          expect(instance).to be_available
+        end
+      end
+
+      context "with the feature flag and the setting disabled",
+              with_flag: { work_package_multiple_versions: false },
+              with_settings: { work_package_multiple_versions: false } do
+        it "is still available" do
+          expect(instance).to be_available
+        end
+      end
+    end
+
+    describe "#valid?" do
+      context "within a project" do
+        context "and the version belongs to the project" do
+          it "is valid" do
+            expect(instance).to be_valid
+          end
+        end
+
+        context "and the version is from another project" do
+          let(:values) { [other_project_version.id.to_s] }
+
+          it "is not valid" do
+            expect(instance).not_to be_valid
+          end
+        end
+      end
+
+      context "without a project" do
+        let(:project) { nil }
+
+        context "and the version is visible to the user" do
+          it "is valid" do
+            expect(instance).to be_valid
+          end
+        end
+
+        context "and the version does not exist" do
+          let(:values) { ["12345"] }
+
+          it "is not valid" do
+            expect(instance).not_to be_valid
+          end
+        end
+      end
+
+      context "with a version status operator and no values" do
+        let(:operator) { "o" }
+        let(:values) { [] }
+
+        it "is valid" do
+          expect(instance).to be_valid
+        end
+      end
+    end
+
+    describe "#allowed_values" do
+      context "within a project" do
+        it "returns the project's shared versions" do
+          expect(instance.allowed_values)
+            .to contain_exactly([version.id.to_s, version.id.to_s])
+        end
+      end
+
+      context "without a project" do
+        let(:project) { nil }
+
+        it "returns only versions visible to the current user" do
+          other_project_version
+
+          expect(instance.allowed_values)
+            .to contain_exactly([version.id.to_s, version.id.to_s])
+        end
+      end
+    end
+
+    describe "#value_objects" do
+      let!(:other_version) { create(:version, project: actual_project) }
+
+      it "returns the Version records matching the filter values" do
+        expect(instance.value_objects).to contain_exactly(version)
+      end
+    end
+
+    describe "#available_operators" do
+      it "includes the version status operators" do
+        expect(instance.available_operators).to include(
+          Queries::Operators::Versions::OpenStatus,
+          Queries::Operators::Versions::ClosedStatus,
+          Queries::Operators::Versions::LockedStatus
+        )
+      end
+    end
+
+    describe "#operator_strategy" do
+      context 'for "o"' do
+        let(:operator) { "o" }
+
+        it "is the open status operator" do
+          expect(instance.operator_strategy).to eq(Queries::Operators::Versions::OpenStatus)
+        end
+      end
+
+      context 'for "c"' do
+        let(:operator) { "c" }
+
+        it "is the closed status operator" do
+          expect(instance.operator_strategy).to eq(Queries::Operators::Versions::ClosedStatus)
+        end
+      end
+
+      context 'for "l"' do
+        let(:operator) { "l" }
+
+        it "is the locked status operator" do
+          expect(instance.operator_strategy).to eq(Queries::Operators::Versions::LockedStatus)
+        end
+      end
+
+      context 'for "="' do
+        it "is the equals operator" do
+          expect(instance.operator_strategy).to eq(Queries::Operators::EqualsOr)
+        end
+      end
+    end
+
+    describe "#where" do
+      let(:open_version) { version }
+      let(:closed_version) { create(:version, project: actual_project, status: "closed") }
+      let(:locked_version) { create(:version, project: actual_project, status: "locked") }
+
+      let!(:wp_observing_open) do
+        create(:work_package, project: actual_project).tap do |wp|
+          create(:work_package_version, work_package: wp, version: open_version, kind: :observed_in)
+        end
+      end
+      let!(:wp_observing_closed) do
+        create(:work_package, project: actual_project).tap do |wp|
+          create(:work_package_version, work_package: wp, version: closed_version, kind: :observed_in)
+        end
+      end
+      let!(:wp_observing_locked) do
+        create(:work_package, project: actual_project).tap do |wp|
+          create(:work_package_version, work_package: wp, version: locked_version, kind: :observed_in)
+        end
+      end
+      # The mirror of the target filter's guard: a target row on the same
+      # version must never be mistaken for an observed one.
+      let!(:wp_targeting_only) do
+        create(:work_package, project: actual_project).tap do |wp|
+          create(:work_package_version, work_package: wp, version: open_version, kind: :target)
+        end
+      end
+      let!(:wp_without_versions) { create(:work_package, project: actual_project) }
+
+      subject(:result) { WorkPackage.where(instance.where) }
+
+      context 'for "=" with a version' do
+        let(:values) { [open_version.id.to_s] }
+
+        it "returns work packages observed in that version" do
+          expect(result).to contain_exactly(wp_observing_open)
+        end
+      end
+
+      context 'for "!" with a version' do
+        let(:operator) { "!" }
+        let(:values) { [open_version.id.to_s] }
+
+        it "returns work packages not observed in that version, including ones without observed versions" do
+          expect(result).to contain_exactly(wp_observing_closed, wp_observing_locked, wp_targeting_only,
+                                            wp_without_versions)
+        end
+      end
+
+      context 'for "*" (any observed version)' do
+        let(:operator) { "*" }
+        let(:values) { [] }
+
+        it "returns work packages with at least one observed version" do
+          expect(result).to contain_exactly(wp_observing_open, wp_observing_closed, wp_observing_locked)
+        end
+      end
+
+      context 'for "!*" (no observed version)' do
+        let(:operator) { "!*" }
+        let(:values) { [] }
+
+        it "returns work packages without any observed version" do
+          expect(result).to contain_exactly(wp_targeting_only, wp_without_versions)
+        end
+      end
+
+      context 'for "o" (open version)' do
+        let(:operator) { "o" }
+        let(:values) { [] }
+
+        it "returns work packages observed in an open version" do
+          expect(result).to contain_exactly(wp_observing_open)
+        end
+      end
+
+      context 'for "c" (closed version)' do
+        let(:operator) { "c" }
+        let(:values) { [] }
+
+        it "returns work packages observed in a closed version" do
+          expect(result).to contain_exactly(wp_observing_closed)
+        end
+      end
+
+      context 'for "l" (locked version)' do
+        let(:operator) { "l" }
+        let(:values) { [] }
+
+        it "returns work packages observed in a locked version" do
+          expect(result).to contain_exactly(wp_observing_locked)
+        end
+      end
+    end
+  end
+end
diff --git a/spec/models/work_packages/pdf_export/work_package_to_pdf_spec.rb b/spec/models/work_packages/pdf_export/work_package_to_pdf_spec.rb
index 2b3bb51b9854..7af9388313dd 100644
--- a/spec/models/work_packages/pdf_export/work_package_to_pdf_spec.rb
+++ b/spec/models/work_packages/pdf_export/work_package_to_pdf_spec.rb
@@ -250,6 +250,7 @@
       "Priority", "Normal",
       *(work_package.sprint.present? ? ["Sprint", work_package.sprint] : ["Sprint"]),
       *(work_package.backlog_bucket.present? ? ["Backlog bucket", work_package.backlog_bucket] : ["Backlog bucket"]),
+      "Observed versions",
       "Version", work_package.version,
       "Category", work_package.category,
       "Project phase",