diff --git a/Lib/concurrent/futures/process.py b/Lib/concurrent/futures/process.py
index 19e93a608b2..695f7733305 100644
--- a/Lib/concurrent/futures/process.py
+++ b/Lib/concurrent/futures/process.py
@@ -400,7 +400,7 @@ class _ExecutorManagerThread(threading.Thread):
         assert not self.thread_wakeup._closed
         wakeup_reader = self.thread_wakeup._reader
         readers = [result_reader, wakeup_reader]
-        worker_sentinels = [p.sentinel for p in self.processes.values()]
+        worker_sentinels = [p.sentinel for p in list(self.processes.values())]
         ready = mp.connection.wait(readers + worker_sentinels)
 
         cause = None
diff --git a/Misc/NEWS.d/next/Library/2021-04-20-14-14-16.bpo-43498.L_Hq-8.rst b/Misc/NEWS.d/next/Library/2021-04-20-14-14-16.bpo-43498.L_Hq-8.rst
new file mode 100644
index 00000000000..4713d1427cc
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-04-20-14-14-16.bpo-43498.L_Hq-8.rst
@@ -0,0 +1,2 @@
+Avoid a possible *"RuntimeError: dictionary changed size during iteration"*
+when adjusting the process count of :class:`ProcessPoolExecutor`.