HMS Avon, later renamed NRP Nuno Tristão, was a River-class frigate of the Royal Navy (RN).

Avon was built to the RN's specifications as a Group II River-class frigate.

The ships were designed by naval engineer William Reed, of Smith's Dock Company of South Bank-on-Tees, to have the endurance and anti-submarine capabilities of the Black Swan-class sloops, while being quick and cheap to build in civil dockyards using the machinery (e.g. reciprocating steam engines instead of turbines) and construction techniques pioneered in the building of the Flower-class corvettes.

After her commission, Avon was deployed in the Indian Ocean, where she rescued the survivors of the sunken Norwegian freighter Tarifa.

[1] The Nuno Tristão was used in several notable events, such as carrying Emperor Haile Selassie of Ethiopia on a state visit from Bayonne to Portugal in July 1959[3] and supporting fuzileiros in Africa.