diff --git a/tox.ini b/tox.ini
index 2e130b7..89d23b3 100644
--- a/tox.ini
+++ b/tox.ini
@@ -26,6 +26,7 @@ requires =
   pip < 20.3
   virtualenv < 20.0
   setuptools < 50.0.0
+  tox < 4.0.0
 
 # NOTE: https://wiki.canonical.com/engineering/OpenStack/InstallLatestToxOnOsci
 minversion = 3.18.0