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 (8,874 cc) displacement, manufactured by Lycoming Engines. The engine is a six-cylinder version of the four-cylinder Lycoming O-360.

Lycoming Engines is a major American manufacturer of aircraft engines. With a factory in Williamsport, Pennsylvania, Lycoming produces a line of horizontally opposed, air-cooled, four, six and eight-cylinder engines including the only FAA-certified aerobatic and helicopter piston engines on the market.