diff --git a/changelog.d/12665.misc b/changelog.d/12665.misc
new file mode 100644
index 000000000000..37b96fea37d3
--- /dev/null
+++ b/changelog.d/12665.misc
@@ -0,0 +1 @@
+Fix spelling of `M_UNRECOGNIZED` in comments.
diff --git a/synapse/federation/federation_client.py b/synapse/federation/federation_client.py
index b5e0b84cbc69..17eff60909a2 100644
--- a/synapse/federation/federation_client.py
+++ b/synapse/federation/federation_client.py
@@ -618,7 +618,7 @@ def _is_unknown_endpoint(
         #
         # Dendrite returns a 404 (with a body of "404 page not found");
         # Conduit returns a 404 (with no body); and Synapse returns a 400
-        # with M_UNRECOGNISED.
+        # with M_UNRECOGNIZED.
         #
         # This needs to be rather specific as some endpoints truly do return 404
         # errors.