The Subaru FA engine is a gasoline boxer-4 engine used in Subaru and Toyota automobiles. It is a derivative of the FB engine, with efforts to reduce weight while maintaining durability as the main design goals. Although the FA and FB engines share a common platform, the FA shares very little in dedicated parts with the FB engine, with a different block, head, connecting rods, and pistons.

A flat-four engine, also known as a horizontally opposed-four engine or boxer engine, is a four-cylinder piston engine with two banks of cylinders lying on opposite sides of a common crankshaft. The most common type of flat-four engine is the boxer-four engine, each pair of opposed pistons moves inwards and outwards at the same time.