diff --git a/README.md b/README.md
index f9bb85454..7eae57940 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,9 @@
 # Mesop: Build delightful web apps quickly in Python 🚀
 
+<!-- prettier-ignore -->
+> [!WARNING]
+> **Mesop is no longer maintained as of September 30, 2026.** No further updates, bug fixes, or support will be provided.
+
 [![PyPI](https://img.shields.io/pypi/v/mesop)](https://pypi.org/project/mesop/)
 [![PyPI downloads](https://img.shields.io/pypi/dm/mesop)](https://pypi.org/project/mesop/)
 [![Twitter follow](https://img.shields.io/twitter/follow/mesop_dev?style=social&label=follow)](https://twitter.com/mesop_dev)
diff --git a/docs/stylesheets/extra.css b/docs/stylesheets/extra.css
index 89b46622a..8b10fc555 100644
--- a/docs/stylesheets/extra.css
+++ b/docs/stylesheets/extra.css
@@ -562,3 +562,14 @@ h3#mesop\.features\.theme\.ThemeVar {
     align-items: center;
   }
 }
+
+/* Archival notice banner (see docs/theme/main.html) */
+.md-banner {
+  background-color: var(--amber-600);
+  color: var(--white);
+}
+
+.md-banner a {
+  color: var(--white);
+  text-decoration: underline;
+}
diff --git a/docs/theme/main.html b/docs/theme/main.html
new file mode 100644
index 000000000..bd912b4a0
--- /dev/null
+++ b/docs/theme/main.html
@@ -0,0 +1,3 @@
+{% extends "base.html" %} {% block announce %}
+<strong>⚠️ Mesop is no longer maintained as of September 30, 2026.</strong>
+No further updates, bug fixes, or support will be provided. {% endblock %}