Index: Lib/threading.py
===================================================================
--- Lib/threading.py	(revision 78514)
+++ Lib/threading.py	(working copy)
@@ -469,7 +469,12 @@
             self._note("%s.start(): starting thread", self)
         with _active_limbo_lock:
             _limbo[self] = self
-        _start_new_thread(self.__bootstrap, ())
+        try:
+            _start_new_thread(self.__bootstrap, ())
+        except Exception:
+            with _active_limbo_lock:
+                del _limbo[self]
+            raise
         self.__started.wait()
 
     def run(self):
Index: Lib/test/test_threading.py
===================================================================
--- Lib/test/test_threading.py	(revision 78514)
+++ Lib/test/test_threading.py	(working copy)
@@ -249,6 +249,25 @@
             t.join()
         # else the thread is still running, and we have no way to kill it
 
+    def test_limbo_cleanup(self):
+        # Issue 7481: Failure to start thread should cleanup the limbo map.
+        def fail_new_thread(*args):
+            raise thread.error()
+        _start_new_thread = threading._start_new_thread
+        threading._start_new_thread = fail_new_thread
+        try:
+            t = threading.Thread(target=lambda: None)
+            try:
+                t.start()
+                assert False
+            except thread.error:
+                self.assertFalse(
+                    t in threading._limbo, 
+                    "Failed to cleanup _limbo map on failure of Thread.start()."
+                )
+        finally:
+            threading._start_new_thread = _start_new_thread
+
     def test_finalize_runnning_thread(self):
         # Issue 1402: the PyGILState_Ensure / _Release functions may be called
         # very late on python exit: on deallocation of a running thread for