The SR U class are 2-6-0 steam locomotives designed by Richard Maunsell for passenger duties on the Southern Railway. The class represented the penultimate stage in the development of the Southern Railway's 2-6-0 "family", which improved upon the basic principles established by GWR CME George Jackson Churchward for GWR locomotives. The U class design drew from experience with the GWR 4300s and N classes, improved by applying Midland Railway ideas to the design, enabling the SECR to influence development of the 2-6-0 in Britain.

The U class was designed in the mid-1920s for production at a time when more obsolete 4-4-0 locomotives were withdrawn and derived from Maunsell's earlier SECR K class 2-6-4 tank locomotives. The first 20 members of the U class were rebuilds of the K class locomotives. A further 20 U class locomotives were built in 1928 to fill the gap in cross-country and semi-fast express passenger services after the withdrawal of the K class. The design also continued the standardisation of the Southern Railway locomotive fleet by using parts designed to be interchangeable with other Maunsell-designed classes.

A total of 50 locomotives were built over three batches between 1928 and 1931, and the design formed the basis for the 3-cylinder U1 class of 1928. They were able to operate over most of the Southern Railway network, gaining the nickname "U-boats" after the submarine warfare of the First World War, and continued to operate with BR. The class saw continuous use until 1966, when all members of the U class were withdrawn from service.

Four U class locomotives have been preserved on two heritage railways in the south of England.