diff --git a/Doc/library/ctypes.rst b/Doc/library/ctypes.rst
--- a/Doc/library/ctypes.rst
+++ b/Doc/library/ctypes.rst
@@ -1022,12 +1022,20 @@
    1 5 7 33 99
    >>>
 
-**Important note for callback functions:**
+**Important notes for callback functions:**
 
 Make sure you keep references to :func:`CFUNCTYPE` objects as long as they are
 used from C code. :mod:`ctypes` doesn't, and if you don't, they may be garbage
 collected, crashing your program when a callback is made.
 
+Also note that if the callback function is called in a new thread that
+has been created outside of Python's control (i.e., by the foreign
+code that calls the callback), ctypes creates a new dummy Python
+thread on every invocation, including recreation of the thread local
+storage area. This behavior is correct for most purposes, but it means
+that values stored with `threading.local` will *not* survive across
+different callbacks, even when those calls are made from the same C
+thread.
 
 .. _ctypes-accessing-values-exported-from-dlls:
 
diff --git a/Misc/NEWS b/Misc/NEWS
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -195,6 +195,9 @@
 - Issue #19286: Directories in ``package_data`` are no longer added to
   the filelist, preventing failure outlined in the ticket.
 
+- Issue #6627: Mention interaction with threading.local() in ctypes
+  documentation. Patch by Nikolaus Rath.
+
 IDLE
 ----