diff --git a/activerecord-spanner-adapter.gemspec b/activerecord-spanner-adapter.gemspec
index 3c1a5c97..c21255aa 100644
--- a/activerecord-spanner-adapter.gemspec
+++ b/activerecord-spanner-adapter.gemspec
@@ -25,6 +25,8 @@ Gem::Specification.new do |spec|
   spec.required_ruby_version = ">= 2.7"
 
   spec.add_dependency "google-cloud-spanner", "~> 2.18"
+  # Pin gRPC to 1.64.3, as 1.65 and 1.66 cause test runs to hang randomly.
+  spec.add_dependency "grpc", "1.64.3"
   spec.add_runtime_dependency "activerecord", [">= 6.0.0", "< 7.2"]
 
   spec.add_development_dependency "autotest-suffix", "~> 1.1"