The LSWR S15 class is a British 2-cylinder 4-6-0 freight steam locomotive designed by Robert W. Urie, based on his H15 class and N15 class locomotives. The class had a complex build history, spanning several years of construction from 1920 to 1936.

The first examples were constructed for the LSWR, where they hauled freight trains to the south coast ports and further west to Exeter, as well as occasional passenger work in conjunction with their larger-wheeled N15 class counterparts. The outline was made during the First World War, and incorporated lessons learned from the operation of his H15 class, a design that was to provide the basis for future standardisation on the LSWR.

To economise on maintenance, the S15 class had interchangeable components that could be used on a similar design, the N15 class passenger locomotive, which had the same overall appearance. Details such as the boiler, the two-cylinders and valve gear were standardised between the classes, although a taper boiler was used on the S15 and N15 classes. The only other major difference was the smaller diameter of the driving wheels. Smaller diameter wheels gave better traction, essential for a fast freight locomotive.

Urie retired as Locomotive Superintendent when the LSWR was amalgamated into the Southern Railway in 1923. Richard Maunsell was given the newly created post of CME to the Southern Railway and decided to revise the cylinder arrangement of the locomotive. In doing so, he delayed the construction of further locomotives until the modifications had been made.

Two Urie S15's have been preserved.