Cowthally Castle, also known as Couthalley, is a ruined L-plan castle near Carnwath, Lanarkshire, Scotland. The remains are protected as a scheduled monument. The castle appears to have been constructed in the 12th century, and was re-built in 1375. The castle passed to Sir John Somerville, 4th Baron of Linton through marriage in 1317 to Lady Elizabeth Douglas. The castle was altered and extended in 1415 and again in 1524. The castle was besieged in 1557 and was partially destroyed and was rebuilt in 1586.