diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index 36297496582..0a78f8c0c22 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -445,6 +445,27 @@ class ThreadTests(BaseTestCase):
         self.assertEqual(out, b'')
         self.assertEqual(err, b'')
 
+    @unittest.skipUnless(hasattr(os, 'fork'), "needs os.fork()")
+    def test_is_alive_after_fork(self):
+        # Try hard to trigger #18418: is_alive() could sometimes be True on
+        # threads that vanished after a fork.
+        old_interval = sys.getswitchinterval()
+        self.addCleanup(sys.setswitchinterval, old_interval)
+
+        # Make the bug more likely to manifest.
+        sys.setswitchinterval(1e-6)
+
+        for i in range(20):
+            t = threading.Thread(target=lambda: None)
+            t.start()
+            self.addCleanup(t.join)
+            pid = os.fork()
+            if pid == 0:
+                os._exit(1 if t.is_alive() else 0)
+            else:
+                pid, status = os.waitpid(pid, 0)
+                self.assertEqual(0, status)
+
 
 class ThreadJoinOnShutdown(BaseTestCase):
 
diff --git a/Lib/threading.py b/Lib/threading.py
index c7f919c33bd..21bc78161f8 100644
--- a/Lib/threading.py
+++ b/Lib/threading.py
@@ -940,7 +940,7 @@ def _after_fork():
     new_active = {}
     current = current_thread()
     with _active_limbo_lock:
-        for thread in _active.values():
+        for thread in _enumerate():
             # Any lock/condition variable may be currently locked or in an
             # invalid state, so we reinitialize them.
             thread._reset_internal_locks()
diff --git a/Misc/ACKS b/Misc/ACKS
index 1eaa897a359..a821cb9b10a 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -288,6 +288,7 @@ Ben Darnell
 Kushal Das
 Jonathan Dasteel
 Pierre-Yves David
+A. Jesse Jiryu Davis
 John DeGood
 Ned Deily
 Vincent Delft
diff --git a/Misc/NEWS b/Misc/NEWS
index 0d7ba2fc318..dfba2cac0c9 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -51,6 +51,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #18418: After fork(), reinit all threads states, not only active ones.
+  Patch by A. Jesse Jiryu Davis.
+
 - Issue #17974: Switch unittest from using getopt to using argparse.
 
 - Issue #11798: TestSuite now drops references to own tests after execution.