M23 is a Morse code station of uncertain origin and purpose. Despite being assigned an ENIGMA designator, it is not a typical number station. Direction finding attempts put the transmitter location in France, possibly near Paris.

M23 is identifiable primarily from its trademark slow speed and transmission lengths of at least 10 minutes. Most broadcasts consist of only a single 3-digit group repeating, ending abruptly without any further traffic. M23 has no long-lived regular schedules; instead, it appears unexpectedly on several time slots, which are active for several days until they suddenly cease.

The rare M23 traffic broadcasts consist of: (1) the 3-digit ID repeated for 10 minutes; (2) the preamble, comprising a "=", the group count (twice), and another "="; (3) the message, made of 5-digit single groups; (4) the repeat separator "= ? ?"; and (5) the end of transmission indicator "AR AR".