The Modified Modular Jack (MMJ) is a small form-factor serial port connector developed by Digital Equipment Corporation (DEC).

As defined by DEC, the six pins are Tx and Rx for the data transmission, their return paths, and DSR and DTR for handshaking.

When connecting two DTE devices such as a computer and a printer, the Digital BC16E crossover cable is used.

[2][failed verification] Thrustmaster is using the MMJ connector for connecting its line of rudder pedals and racing pedals to either USB via a proprietary Thrustmaster Adapter, or directly to a joystick or racing wheel.

[citation needed] The Lego Mindstorms NXT and EV3 set used connectors similar to MMJ for motors and sensors.