diff --git a/contrib/opencensus-ext-azure/setup.py b/contrib/opencensus-ext-azure/setup.py
index 28d8c450a..54a1c3fea 100644
--- a/contrib/opencensus-ext-azure/setup.py
+++ b/contrib/opencensus-ext-azure/setup.py
@@ -46,6 +46,7 @@
         'opencensus >= 0.12.dev0, < 1.0.0',
         'psutil >= 5.6.3',
         'requests >= 2.19.0',
+        'protobuf <= 3.20.*'
     ],
     extras_require={},
     license='Apache-2.0',