diff --git a/gems/smithy-client/lib/smithy-client.rb b/gems/smithy-client/lib/smithy-client.rb
index 254e67b76..2a4f8bb04 100644
--- a/gems/smithy-client/lib/smithy-client.rb
+++ b/gems/smithy-client/lib/smithy-client.rb
@@ -32,6 +32,8 @@
 require_relative 'smithy-client/param_validator'
 require_relative 'smithy-client/plugin'
 require_relative 'smithy-client/plugin_list'
+require_relative 'smithy-client/protocol'
+require_relative 'smithy-client/no_op_protocol'
 require_relative 'smithy-client/retry'
 require_relative 'smithy-client/service_error'
 require_relative 'smithy-client/util'
diff --git a/gems/smithy-client/lib/smithy-client/no_op_protocol.rb b/gems/smithy-client/lib/smithy-client/no_op_protocol.rb
new file mode 100644
index 000000000..9fc15fc59
--- /dev/null
+++ b/gems/smithy-client/lib/smithy-client/no_op_protocol.rb
@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+require_relative 'protocol'
+
+module Smithy
+  module Client
+    # Default protocol used when no protocol is registered for a client. All
+    # methods are no-ops, so build/parse handlers can delegate safely without
+    # a nil check.
+    # @api private
+    class NoOpProtocol < Protocol
+      def build_request(_context); end
+
+      def parse_data(_response); end
+
+      def parse_error(_response); end
+    end
+  end
+end
diff --git a/gems/smithy-client/lib/smithy-client/plugins/protocol.rb b/gems/smithy-client/lib/smithy-client/plugins/protocol.rb
new file mode 100644
index 000000000..1eb371380
--- /dev/null
+++ b/gems/smithy-client/lib/smithy-client/plugins/protocol.rb
@@ -0,0 +1,68 @@
+# frozen_string_literal: true
+
+require_relative '../protocol'
+require_relative '../no_op_protocol'
+
+module Smithy
+  module Client
+    module Plugins
+      # Generic protocol plugin. Resolves the +:protocol+ option and installs
+      # build/parse handlers that delegate serialization and deserialization to
+      # the resolved protocol instance.
+      # @api private
+      class Protocol < Plugin
+        option(
+          :protocol,
+          doc_type: 'Symbol, Object',
+          docstring: 'The protocol used for request serialization and response ' \
+                     'deserialization. Defaults to the service default protocol. ' \
+                     'May be a Symbol naming a supported protocol, or a custom ' \
+                     'object implementing the protocol interface.'
+        )
+
+        # @api private
+        class BuildHandler < Handler
+          def call(context)
+            context.config.protocol.build_request(context)
+            @handler.call(context)
+          end
+        end
+
+        # @api private
+        class ParseHandler < Handler
+          def call(context)
+            response = @handler.call(context)
+            response.error = context.config.protocol.parse_error(response) unless response.error
+            response.data = context.config.protocol.parse_data(response) unless response.error
+            response
+          end
+        end
+
+        def add_handlers(handlers, _config)
+          handlers.add(BuildHandler)
+          handlers.add(ParseHandler)
+        end
+
+        def before_initialize(client_class, options)
+          case options[:protocol]
+          when nil
+            options[:protocol] = client_class.protocols.values.first&.new || NoOpProtocol.new
+          when Symbol
+            options[:protocol] = resolve!(client_class, options[:protocol])
+            # else: custom protocol instance, used as-is
+          end
+        end
+
+        private
+
+        # TODO: client class to have prioritized list
+        def resolve!(client_class, name)
+          protocol_class = client_class.protocols[name]
+          raise ArgumentError, "Unknown protocol: #{name}" unless protocol_class
+
+          protocol_class.new
+        end
+      end
+    end
+  end
+end
diff --git a/gems/smithy-client/lib/smithy-client/protocol.rb b/gems/smithy-client/lib/smithy-client/protocol.rb
new file mode 100644
index 000000000..5bbedb7a2
--- /dev/null
+++ b/gems/smithy-client/lib/smithy-client/protocol.rb
@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+module Smithy
+  module Client
+    # Base class documenting the protocol interface. A protocol serializes
+    # requests and deserializes responses for a specific wire format.
+    #
+    # A custom protocol passed via +Client.new(protocol:)+ is any object that
+    # responds to these methods; it need not inherit from this class.
+    # @api private
+    class Protocol
+      # Serialize the request into the wire format.
+      # @param [Interceptor::Context] _context
+      def build_request(_context)
+        raise NotImplementedError
+      end
+
+      # Deserialize a successful response body.
+      # @param [Response] _response
+      def parse_data(_response)
+        raise NotImplementedError
+      end
+
+      # Deserialize an error response into the modeled error.
+      # @param [Response] _response
+      def parse_error(_response)
+        raise NotImplementedError
+      end
+    end
+  end
+end
diff --git a/gems/smithy-client/spec/smithy-client/plugins/protocol_spec.rb b/gems/smithy-client/spec/smithy-client/plugins/protocol_spec.rb
new file mode 100644
index 000000000..34615a6bf
--- /dev/null
+++ b/gems/smithy-client/spec/smithy-client/plugins/protocol_spec.rb
@@ -0,0 +1,120 @@
+# frozen_string_literal: true
+
+require_relative '../../spec_helper'
+
+require 'smithy-client/plugins/protocol'
+
+module Smithy
+  module Client
+    module Plugins
+      describe Protocol do
+        let(:sample_client) { ClientHelper.sample_client }
+        let(:client_class) do
+          client_class = sample_client.const_get(:Client)
+          client_class.clear_plugins
+          client_class.add_plugin(sample_client::Plugins::Endpoint)
+          client_class.add_plugin(Protocol)
+          client_class
+        end
+        let(:client_options) { { endpoint: 'https://example.com' } }
+
+        let(:fake_protocol_class) do
+          Class.new(Client::Protocol) do
+            def build_request(_context); end
+            def parse_data(_response); end
+            def parse_error(_response); end
+          end
+        end
+
+        # TODO: remove this test-only registry once the `protocols` weld is impl
+        before do
+          protocols = { rpc_v2_cbor: fake_protocol_class }
+          client_class.define_singleton_method(:protocols) { protocols }
+        end
+
+        it 'adds a :protocol option to config' do
+          client = client_class.new(client_options)
+          expect(client.config).to respond_to(:protocol)
+        end
+
+        context 'when :protocol is not provided (default)' do
+          it 'defaults to an instance of the first registered protocol' do
+            client = client_class.new(client_options)
+            expect(client.config.protocol).to be_a(fake_protocol_class)
+          end
+
+          it 'falls back to NoOpProtocol when no protocol is registered' do
+            client_class.define_singleton_method(:protocols) { {} }
+            client = client_class.new(client_options)
+            expect(client.config.protocol).to be_a(Client::NoOpProtocol)
+          end
+        end
+
+        context 'when :protocol is a Symbol' do
+          it 'resolves to an instance of the named protocol' do
+            client = client_class.new(client_options.merge(protocol: :rpc_v2_cbor))
+            expect(client.config.protocol).to be_a(fake_protocol_class)
+          end
+
+          it 'raises an ArgumentError for an unknown protocol' do
+            expect { client_class.new(client_options.merge(protocol: :nope)) }
+              .to raise_error(ArgumentError, /Unknown protocol: nope/)
+          end
+        end
+
+        context 'when :protocol is a custom object' do
+          it 'uses the provided object as-is' do
+            custom = fake_protocol_class.new
+            client = client_class.new(client_options.merge(protocol: custom))
+            expect(client.config.protocol).to be(custom)
+          end
+        end
+
+        it 'adds the build and parse handlers' do
+          client = client_class.new(client_options)
+          expect(client.handlers).to include(Protocol::BuildHandler)
+          expect(client.handlers).to include(Protocol::ParseHandler)
+        end
+      end
+
+      describe Client::Protocol do
+        subject(:protocol) { described_class.new }
+
+        it 'raises NotImplementedError for #build_request' do
+          expect { protocol.build_request(double('context')) }
+            .to raise_error(NotImplementedError)
+        end
+
+        it 'raises NotImplementedError for #parse_data' do
+          expect { protocol.parse_data(double('response')) }
+            .to raise_error(NotImplementedError)
+        end
+
+        it 'raises NotImplementedError for #parse_error' do
+          expect { protocol.parse_error(double('response')) }
+            .to raise_error(NotImplementedError)
+        end
+      end
+
+      describe Client::NoOpProtocol do
+        subject(:protocol) { described_class.new }
+
+        it 'is a Protocol' do
+          expect(protocol).to be_a(Client::Protocol)
+        end
+
+        it 'returns nil from #build_request without raising' do
+          expect(protocol.build_request(double('context'))).to be_nil
+        end
+
+        it 'returns nil from #parse_data without raising' do
+          expect(protocol.parse_data(double('response'))).to be_nil
+        end
+
+        it 'returns nil from #parse_error without raising' do
+          expect(protocol.parse_error(double('response'))).to be_nil
+        end
+      end
+    end
+  end
+end