diff --git a/cirq-google/requirements.txt b/cirq-google/requirements.txt
index 48a10d90917..3616edade5a 100644
--- a/cirq-google/requirements.txt
+++ b/cirq-google/requirements.txt
@@ -1,3 +1,3 @@
 google-api-core[grpc] >= 1.14.0, < 2.0.0dev
 proto-plus>=1.20.0
-protobuf>=3.13.0
+protobuf>=3.13.0<4