diff --git a/tox.ini b/tox.ini
index 71eaa3d024..882a66383d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -6,6 +6,7 @@ envlist = py3,pep8
 # env and ignore basepython inherited from [testenv] if we set
 # ignore_basepython_conflict.
 ignore_basepython_conflict = true
+requires = tox<4
 
 [testenv]
 basepython = python3
diff --git a/zuul.d/project.yaml b/zuul.d/project.yaml
index b33dcbeb4f..3af4625c9f 100644
--- a/zuul.d/project.yaml
+++ b/zuul.d/project.yaml
@@ -1,4 +1,6 @@
 - project:
+    vars:
+      ensure_tox_version: '<4'
     templates:
       - publish-openstack-docs-pti
       - openstack-cover-jobs