diff --git a/tox.ini b/tox.ini
index e70aef2d2..69f52bd9f 100644
--- a/tox.ini
+++ b/tox.ini
@@ -18,6 +18,8 @@ deps =
     # Extras
     botocore>=1.4.87
     Pillow>=4.0.0
+    # Twisted 21+ causes issues in tests that use skipIf
+    Twisted<21
 passenv =
     S3_TEST_FILE_URI
     AWS_ACCESS_KEY_ID