Arx, ARX, or ArX may refer to:
- ARX (operating system), an operating system
- Arx, the aristaless related homeobox gene and protein
- Arx, Landes, a commune of the Landes département in France
- ArX (revision control), revision control software
- ObjectARX, a software API for AutoCAD
- Arx (Roman), a Roman citadel, and in particular:
- The northern hump of the two forming the Capitoline Hill of ancient Rome
- Arx Fatalis, a first person role-playing game developed by Arkane Studios in 2002
- Arx, one of Lars Vilks sculptures