diff --git a/setup.py b/setup.py
index 19f092686..9fb99c111 100644
--- a/setup.py
+++ b/setup.py
@@ -32,7 +32,7 @@
     "grpcio >= 1.38.1, < 2.0dev",  # https://github.com/googleapis/python-pubsub/issues/414
     "google-api-core[grpc] >= 1.32.0, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.*,!=2.4.*,!=2.5.*,!=2.6.*,!=2.7.*",
     "proto-plus >= 1.22.0, <2.0.0dev",
-    "protobuf >= 3.20.2, <5.0.0dev",
+    "protobuf>=3.19.5,<5.0.0dev,!=3.20.0,!=3.20.1,!=4.21.0,!=4.21.1,!=4.21.2,!=4.21.3,!=4.21.4,!=4.21.5",
     "grpc-google-iam-v1 >=0.12.4, <1.0.0dev",
     "grpcio-status >= 1.16.0",
 ]
diff --git a/testing/constraints-3.7.txt b/testing/constraints-3.7.txt
index 22fedb9e7..08b242a12 100644
--- a/testing/constraints-3.7.txt
+++ b/testing/constraints-3.7.txt
@@ -9,4 +9,4 @@ google-api-core==1.32.0
 libcst==0.3.10
 proto-plus==1.22.0
 grpc-google-iam-v1==0.12.4
-protobuf==3.20.2
+protobuf==3.19.5