The Alvis Leonides is a British air-cooled nine-cylinder radial aero engine first developed by Alvis Car and Engineering Company in 1936.

The prototype engine, called 9ARS and which weighed 693 lb and developed 450 hp, was run in December 1936.

(Most helicopter engines were direct drive — no reduction gearbox — with a centrifugal clutch and fan cooling).

It was Britain's last high-power production piston aero-engine when manufacture ceased in 1966.

Preserved Alvis Leonides engines are on public display at the following museums: