Saint Thomas, city in southern Ontario, Canada, seat of Elgin County, on Kettle Creek, near Lake Erie. Saint Thomas is a commercial and light-manufacturing center situated in a rich agricultural area. Major products include motor-vehicle parts; machinery; metal, glass, and wood items; and chemicals. The community, established in 1810, later grew as a railroad center; it was incorporated in 1881. Population 28,943 (1986); 32,275 (1996).