diff --git a/src/urllib3/contrib/_appengine_environ.py b/src/urllib3/contrib/_appengine_environ.py
index 119efaeeb6..38e227b2e9 100644
--- a/src/urllib3/contrib/_appengine_environ.py
+++ b/src/urllib3/contrib/_appengine_environ.py
@@ -6,7 +6,9 @@
 
 
 def is_appengine():
-    return "APPENGINE_RUNTIME" in os.environ
+    return "APPENGINE_RUNTIME" in os.environ and not os.environ.get(
+        "SERVER_SOFTWARE", ""
+    ).startswith("testutil/")
 
 
 def is_appengine_sandbox():