diff --git a/doc/conf.py b/doc/conf.py
index a82c0e03..7beafddc 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -88,10 +88,14 @@ extensions = [
     "sphinx.ext.extlinks",
 ]
 
-# The search extension works only on readthedocs
-# See https://readthedocs-sphinx-search.readthedocs.io
 if on_rtd:
-    extensions.append("sphinx_search.extension")
+    extensions += [
+        # The custom 404 page is only needed
+        "notfound.extension",
+        # The search extension works only on readthedocs
+        # See https://readthedocs-sphinx-search.readthedocs.io
+        "sphinx_search.extension",
+    ]
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = [doc_path + '/_templates']
diff --git a/doc/requirements.txt b/doc/requirements.txt
index f31b40c3..bf95a390 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -1,2 +1,3 @@
 sphinx-rtd-theme~=1.3.0
+sphinx-notfound-page~=1.0.0
 readthedocs-sphinx-search~=0.3.1