The Avro 521 was a British two-seat fighter first flown late in 1915, based on the 504. Only a prototype of the Avro 521 was built. It was powered by a 110 hp (80 kW) Clerget engine, with a provision for a .303 in (7.7 mm) Lewis Gun in the rear cockpit.
The prototype underwent trials with the RFC in early 1916, and 25 aircraft were subsequently ordered. However, this contract was cancelled, and there is no evidence of any other Avro 521s being built. The prototype crashed at Central Flying School Upavon on 21 September 1916, killing pilot Lieutenant W.H.S. Garnett.