diff --git a/ortools/python/setup.py.in b/ortools/python/setup.py.in
index f1e1babe36..e0627a3aea 100644
--- a/ortools/python/setup.py.in
+++ b/ortools/python/setup.py.in
@@ -46,7 +46,7 @@ setup(
         'absl-py >= 2.0.0',
         'numpy >= 1.13.3',
         'pandas >= 2.0.0',
-        'protobuf >= 5.26.1',
+        'protobuf >= 5.26.1,<5.27',
         'immutabledict >= 3.0.0',
     ],
     package_data={