city, seat of St. Louis county and a suburb of St. Louis, eastern Missouri, U.S. Founded in 1876, it was named for Ralph Clayton, a farmer from Virginia who had settled in the area in the 1830s and donated land for the establishment of a county seat after the city of St. Louis elected to separate from the county. Another native Virginian, Martin Hanley, arrived during the same period and also gave land to the town. The new county courthouse, formerly located within St. Louis city limits, was built on Clayton and Hanley's donated property.

At first the town was dominated by economic activity surrounding the courts, but within a few years a school, a newspaper, and trolley service were established. Well-to-do St. Louisans began relocating to Clayton, and the population grew rapidly. The first city hall was built in 1931. In the 1940s a building and economic boom began that by the late 1950s started to include high-rise buildings downtown.

Today Clayton, while principally residential, is a major county retail and commercial centre. The city has a symphony orchestra and maintains the historic Martin Franklin Hanley House, the farmhouse built by Hanley in 1855. Higher-education institutions include Concordia Theological Seminary (1839) and Fontbonne College (1917). Inc. 1913. Pop. (2000) 12,825; (2005 est.) 16,061.