The Airspeed AS.51 Horsa was a British troop-carrying glider used during the Second World War. It was developed and manufactured by Airspeed Limited, alongside various subcontractors; the type was named after Horsa, the legendary 5th-century conqueror of southern Britain.

Having been greatly impressed by the effective use of airborne operations by Germany during the early stages of the Second World War, such as during the Battle of France, the Allied powers sought to establish capable counterpart forces of their own. The British War Office, determining that the role of gliders would be an essential component of such airborne forces, proceeded to examine available options. An evaluation of the General Aircraft Hotspur found it to lack the necessary size, thus Specification X.26/40 was issued. It was from this specification that Airspeed Limited designed the Horsa, a large glider capable of accommodating up to 30 fully equipped troops, which was designated as the AS 51.