diff --git a/.github/workflows/docs.yaml b/.github/workflows/docs.yaml
index e1a4fbe..e0b4dbd 100644
--- a/.github/workflows/docs.yaml
+++ b/.github/workflows/docs.yaml
@@ -105,6 +105,11 @@ jobs:
             echo '<meta http-equiv="refresh" content="0; url=doc/stable/" />' > pages/index.html
           fi
 
+          # File not found (404) page from repo.
+          if [[ -f www/404.html ]]; then
+            cp www/404.html pages/404.html
+          fi
+
       - name: Setup Pages
         uses: actions/configure-pages@45bfe0192ca1faeb007ade9deae92b16b8254a0d # v6.0.0
 
diff --git a/www/404.html b/www/404.html
new file mode 100644
index 0000000..c062355
--- /dev/null
+++ b/www/404.html
@@ -0,0 +1,40 @@
+<!DOCTYPE html>
+<html lang="en">
+
+<head>
+  <meta charset="UTF-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1.0">
+  <title>404 Not Found — spdx-python-model</title>
+  <meta name="description" content="Page not found.">
+  <meta http-equiv="refresh" content="3; url=/spdx-python-model/">
+  <style>
+    body {
+      font-family: system-ui, sans-serif;
+      max-width: 640px;
+      margin: 4rem auto;
+      padding: 0 1.5rem;
+      color: #222;
+    }
+
+    h1 {
+      margin-bottom: 0.25rem;
+    }
+
+    p.tagline {
+      color: #555;
+      margin-top: 0;
+    }
+
+    a {
+      color: #0969da;
+    }
+  </style>
+</head>
+
+<body>
+  <h1>404 — Page Not Found</h1>
+  <p class="tagline">The page you requested does not exist.</p>
+  <p>Go back to the <a href="/spdx-python-model/">spdx-python-model home page</a>.</p>
+</body>
+
+</html>
