diff --git a/.zuul.yaml b/.zuul.yaml
index e29487bf1..64d85ead0 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -38,6 +38,8 @@
       tox_envlist: functional-py39
 
 - project:
+    vars:
+      ensure_tox_version: '<4'
     templates:
       - check-requirements
       - lib-forward-testing-python3
diff --git a/tox.ini b/tox.ini
index cda4d2673..1cd674d12 100644
--- a/tox.ini
+++ b/tox.ini
@@ -2,6 +2,7 @@
 distribute = False
 envlist = py3,pep8
 minversion = 3.18.0
+requires = tox<4
 skipsdist = True
 # this allows tox to infer the base python from the environment name
 # and override any basepython configured in this file