diff --git a/Lib/pydoc.py b/Lib/pydoc.py
index 376c4459fe6..5edc77005a8 100644
--- a/Lib/pydoc.py
+++ b/Lib/pydoc.py
@@ -2297,6 +2297,10 @@ def ready(self, server):
         def stop(self):
             """Stop the server and this thread nicely"""
             self.docserver.quit = True
+            self.join()
+            # explicitly break a reference cycle: DocServer.callback
+            # has indirectly a reference to ServerThread.
+            self.docserver = None
             self.serving = False
             self.url = None
 
diff --git a/Misc/NEWS.d/next/Library/2017-08-21-12-31-53.bpo-31238.Gg0LRH.rst b/Misc/NEWS.d/next/Library/2017-08-21-12-31-53.bpo-31238.Gg0LRH.rst
new file mode 100644
index 00000000000..3e1cd130100
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2017-08-21-12-31-53.bpo-31238.Gg0LRH.rst
@@ -0,0 +1,3 @@
+pydoc: the stop() method of the private ServerThread class now waits until
+DocServer.serve_until_quit() completes and then explicitly sets its
+docserver attribute to None to break a reference cycle.