MJ may refer to:
Mjolnir () is a fictional mythical weapon appearing in American comic books published by Marvel Comics.
mjunction services limited is a 50:50 joint venture of two Indian steel conglomerate – the Steel Authority of India Ltd (SAIL) and Tata Steel.
Mjøsa is Norway's largest lake, as well as one of the deepest lakes in Norway and in Europe. It is the fourth-deepest lake in Norway.
MJM Music PL, is a Polish independent record label founded in 1990 in Warsaw by Polskie Radio journalist Andrzej Wojciechowski.
Mjötviður Mær, released in 1981, was the second album by Icelandic new wave/rock group Þeyr. It was edited through Eskvímó in 12” vinyl.
Mjällby Allmänna Idrottsförening, also known simply as Mjällby AIF, Mjällby or (especially locally) MAIF, is a Swedish professional football club based in Hällevik.