diff --git a/setup.py b/setup.py
index abd5cef95..c7410601e 100644
--- a/setup.py
+++ b/setup.py
@@ -34,6 +34,7 @@
     "google-cloud-core >= 1.4.1, < 2.0dev",
     "google-resumable-media >= 0.6.0, < 2.0dev",
     "six >=1.13.0,< 2.0.0dev",
+    "protobuf >= 3.12.0",
 ]
 extras = {
     "bqstorage": [
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index 798804941..cea0ed84e 100644
--- a/testing/constraints-3.6.txt
+++ b/testing/constraints-3.6.txt
@@ -8,6 +8,7 @@ libcst==0.2.5
 llvmlite==0.34.0
 # pandas 0.23.0 is the first version to work with pyarrow to_pandas.
 pandas==0.23.0
+protobuf == 3.12.0
 proto-plus==1.10.0
 pyarrow==1.0.0
 python-snappy==0.5.4