Auxiliary may refer to:
- A backup site or system
- Auxiliary input jack, generally for audio
- Auxiliary verb
- International auxiliary language
- Auxiliary police
- Auxiliaries, troops supporting the main force of an army
- Auxiliaries (Roman military)
- Auxiliary route, also known as "special route", in road transportation
- An auxiliary route of the Interstate Highway System in the United States
- Auxiliary bishop, in the Roman Catholic Church
- Auxiliary organization in the Latter Day Saint movement
- Aux-send of a mixing console
- A marching band colorguard