diff --git a/tox.ini b/tox.ini
index 6df34e5..b1edf7d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -11,6 +11,8 @@ envlist = pep8,py3
 sitepackages = False
 # NOTE: Avoid false positives by not skipping missing interpreters.
 skip_missing_interpreters = False
+requires =
+  tox < 4.0.0
 
 # NOTE: https://wiki.canonical.com/engineering/OpenStack/InstallLatestToxOnOsci
 minversion = 3.18.0