diff --git a/test-requirements.txt b/test-requirements.txt
index e93a02978..edf088ad7 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -17,5 +17,6 @@ keystoneauth1
 oslo.config
 python-novaclient
 tenacity
+protobuf<3.21.0
 # Fix upper version to ensure compatibility with Juju 3.1
 juju<3.2