diff --git a/setup.py b/setup.py
index e55afad..9df75d8 100644
--- a/setup.py
+++ b/setup.py
@@ -48,7 +48,7 @@
         # https://github.com/googleapis/google-cloud-python/issues/10566
         "google-api-core[grpc] >= 1.31.5, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.0",
         "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",
     ),
     python_requires=">=3.7",
diff --git a/testing/constraints-3.7.txt b/testing/constraints-3.7.txt
index b350a8a..4d63535 100644
--- a/testing/constraints-3.7.txt
+++ b/testing/constraints-3.7.txt
@@ -8,4 +8,4 @@
 google-api-core==1.31.5
 proto-plus==1.22.0
 grpc-google-iam-v1==0.12.4
-protobuf==3.20.2
+protobuf==3.19.5