The LNER Gresley A3 class locomotives represented two distinct stages in the history of the British 4-6-2 "Pacific" steam locomotives designed by Nigel Gresley. As A1's they were designed for main line passenger services and later express passenger services, initially on the GNR, a constituent company of the LNER after the amalgamation of 1923, for which they became a standard design.

The change in class designation to A3 reflected the fitting to the same chassis of a higher-pressure boiler with a greater superheating surface and a small reduction in cylinder diameter, leading to an increase in locomotive weight. Eventually all of the A1 locomotives were rebuilt, most to A3 specifications. One was named after the company's most famous long-distance passenger train, the Flying Scotsman. 78 were built, but Flying Scotsman is the sole member of the class to be preserved.