MJ may refer to:
- Manufacturers' Junction Railway's reporting mark
- Líneas Aéreas Privadas Argentinas' IATA code
- Jeep Comanche or MJ, a pickup truck
- Millijoule (mJ) or megajoule (MJ), units of energy
- MJ, a defunct New York City subway service
- MJ, Jupiter mass, a unit of mass
- Master of Jurisprudence, a graduate law degree
- MJ, citation abbreviation for West's Military Justice Reporter
- Marijuana
Read more about MJ: People