diff --git a/tox.ini b/tox.ini
index db751b5b4..adb0374f3 100644
--- a/tox.ini
+++ b/tox.ini
@@ -157,6 +157,9 @@ commands=
 [testenv:mypy]
 deps =
     {[base]deps}
+    # Type hints are broken with Twisted > 20.3.0, see https://github.com/matrix-org/synapse/issues/9513
+    # TODO: Remove after merging in the fixes from mainline
+    twisted==20.3.0
 extras = all,mypy
 commands = mypy