HJ may refer to:

Science, technology, and mathematics

* Hall–Janko group, a mathematical group * U.S. code for a cryptographic key change; see cryptoperiod

Other uses

* , a two-letter combination used in some languages ** /hj/, a pronunciation cluster, sometimes reduced * Holden HJ, an Australian car 1974-1976 * Hajji (Hj.), an Islamic honorific * ''hic jacet'' ('here lies'), Latin phrase on gravestones * Hilal-i-Jurat, post-nominal for Pakistan honour * Hitler-Jugend (Hitler Youth) * Tasman Cargo Airlines, IATA airline designator * Handjob {{disambiguation