diff --git a/setup.py b/setup.py
index 0a42bd1..d5414c3 100644
--- a/setup.py
+++ b/setup.py
@@ -40,7 +40,8 @@
     include_package_data=True,
     install_requires=(
         "google-api-core[grpc] >= 1.31.5, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.0",
-        "proto-plus >= 1.15.0",
+        "proto-plus >= 1.15.0, <2.0.0dev",
+        "protobuf >= 3.19.0, <4.0.0dev",
     ),
     python_requires=">=3.6",
     classifiers=[
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index 9f829f6..3b53664 100644
--- a/testing/constraints-3.6.txt
+++ b/testing/constraints-3.6.txt
@@ -8,3 +8,4 @@
 google-api-core==1.31.5
 proto-plus==1.15.0
 libcst==0.2.5
+protobuf==3.19.0