diff --git a/setup.py b/setup.py
index 62636e4..95a3ed7 100644
--- a/setup.py
+++ b/setup.py
@@ -29,7 +29,7 @@
 release_status = "Development Status :: 4 - Beta"
 dependencies = [
     "google-api-core[grpc] >= 1.31.5, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.0",
-    "protobuf >= 3.12.0",
+    "protobuf >= 3.12.0, <4.0.0dev",
 ]
 
 # Setup boilerplate below this line.