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
• MMJ, Filipino singing duo [https://tl.wikipedia.org/wiki/MMJ]

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