LJ may refer to:
- Sierra National Airlines' IATA code
- LJ, the sequent calculus of Gentzen's for intuitionist logic
- LiveJournal
- Linux Journal
- La Jolla, California
- Library Journal
- Lightweight Java
- Ljubljana, the capital of Slovenia
- LaserJet
- Lord Justice of Appeal (plural: LJJ)
- Larry Johnson (basketball, born 1969)
- Lawrence Jackson, USC defensive lineman
- Larry Johnson (American football)
- LJ Reyes
- LJ Create
- Lennard-Jones potential, a function used in chemical physics
- Lemon Jelly, a British electronica duo
- Lashkar i Jhangvi, Muslim terrorist organization
Read more about LJ: Fictional Characters