A flat-6 or horizontally-opposed-6 is a flat engine with six cylinders arranged horizontally in two banks of three cylinders on each side of a central crankcase. The pistons are mounted to the crankshaft such that opposing pistons move back and forth in opposite directions at the same time, somewhat like a boxing competitor punching his gloves together before a fight, which has led to it being referred to as a boxer engine.

The configuration results in inherently good balance of the reciprocating parts, a low center of gravity, and a very short engine length. The layout also lends itself to efficient air cooling. However, it is an intrinsically expensive design to manufacture, and somewhat too wide for compact automobile engine compartments, which makes it more suitable for luxury sports cars, cruising motorcycles, and aircraft than ordinary passenger cars.

The shape of the engine suits it better for rear engine and mid-engine designs, where the low center of gravity is an advantage; in front engine designs the width interferes with the ability of the front wheels to steer. Only a few auto makers, including Porsche and Subaru, currently use horizontally opposed engines. Porsche continues to be the most prominent manufacturer of flat-6 engine luxury sports cars, while Subaru uses it in its more expensive four-wheel drive cars, where the difficulties of fitting the engine between the front wheels are offset by the efficiency of adding four-wheel drive to the layout. In the past a number of other manufacturers have used them, notably Tucker in the 1948 Tucker Sedan, Citroën in the DS, and Chevrolet in the 1960s Corvair with flat-6 air-cooled engines.