The Liberty L-6 was a six-cylinder water-cooled inline aircraft engine developed in the United States during World War I.

The Liberty L-6, which developed 200–215 hp, was built by the Thomas-Morse Aircraft Corp. and Wright Aeronautical Corp.

Since it was based on the same engine design as the more successful Liberty L-12 V-12 liquid-cooled aviation engine, the L-6's resemblance to the Mercedes D.III German aviation engine, the source for the Liberty V-12's own cylinder and valvetrain design, resulted in the American L-6 engine design bearing a close visual resemblance to the German straight-six aviation powerplant in a number of respects, with at least one L-6 even being mounted postwar into a captured Fokker D.VII fighter for testing in the US.

(13.5 liters) versus the late-war German Mercedes D.IIIaü's 903 cubic inches (14.8 liters) do not seem to have handicapped the American straight-six design, however, as the "Liberty Six" possessed a 5.42:1 CR, while the D.IIIaü had only a 4.64:1 CR, explaining a good bit of the American powerplant's output level—in addition to the 735.5 watt level for German Pferdestärke metric horsepower, versus the then solely-American-based SAE organization's standard of almost 746 watts per one horsepower.

Data from U.S. Army Air Service Information Circular – Performance Test of Fokker D-VII with Liberty Six Engine Comparable engines Related lists This article incorporates text from Liberty L-6, a public domain work of the United States Government.