The Lycoming O-540 is a family of air-cooled six-cylinder, horizontally opposed fixed-wing aircraft and helicopter engines of 541.5 cubic inches (8.9 L) displacement, manufactured by Lycoming Engines.

Producing between 230 and 350 horsepower (170 and 260 kW)[1] these engines are installed in a large variety of aircraft.

An AEIO version was developed for high-performance competition aerobatics aircraft.

The AEIO-540 family has achieved considerable success in aircraft such as the Extra 300, CAP 232, and Zivko Edge 540.

[1] The numerous engine suffixes denote different accessories such as different manufacturers' carburetors, or different magnetos.