Reactor can mean:
- Bioreactor, any device or system that supports a biologically active environment.
- Chemical reactor, a device for containing and controlling a chemical reaction
- Nuclear reactor, a device for containing and controlling a nuclear reaction
- Fusion reactor, a device for containing and controlling a fusion power reaction
- An inductor (possessing reactance) in an electrical power grid
- A Line reactor is used to limit starting current of motors and to protect variable frequency drives
- The reactor pattern, a design pattern used in concurrent programming
- Reactor (arcade game), an arcade game created by Gottlieb
- Reactor, Inc., a defunct interactive entertainment company founded by Mike Saenz
- Reactor (Havok), a physics simulation engine
- Re-ac-tor, a 1981 album by Neil Young and Crazy Horse