HJ may refer to:

Science, technology, and mathematics

  • Hall–Janko group, a mathematical group
  • Hamilton–Jacobi equation, an equation in classical mechanics which also has relations to quantum physics
  • U.S. code for a cryptographic key change; see cryptoperiod

Other uses

  • List of Latin-script digraphs#H|, a two-letter combination used in some languages
  • hj-reduction in English, dropping the sound before
  • Hajji (Hj.), an Islamic honorific
  • Handjob
  • hic jacet ('here lies'), Latin phrase on gravestones
  • Hilal-i-Jurat, post-nominal for Pakistan honour
  • Hitler-Jugend (Hitler Youth)
  • Holden HJ, an Australian car 1974-1976
  • Hot Jupiter, a type of planet
  • Tasman Cargo Airlines, IATA airline designator
  • Jiaguwen Heji, the standard comprehensive collection of rubbings of ancient Chinese oracle bone inscriptions
  • "High jump" athletics abbreviation in track and field