MMJ is a three-letter abbreviation which may represent:
- Medical Marijuana or Medicinal Marijuana
- the IATA airport code of Matsumoto Airport in Matsumoto, Nagano Prefecture, Japan
- Moscow Mathematical Journal, a quarterly international academic journal published by the Independent University of Moscow
- the ISO 639-3 identifier code for the Majhwar language
- Modified Modular Jack, a six-pin modular connector used by Digital Equipment Corporation on their computers and peripherals.
- My Morning Jacket, an alternative rock band from Kentucky
- Multi-Media Journalist