State Route 309 (SR 309) is an east–west highway in central Ohio. Its western terminus is at its interchange with U.S. 30 near Delphos, Ohio, and its eastern terminus is at its interchange with U.S. 30 in Mansfield, Ohio. Its current route takes it through the cities of Lima, Kenton, Marion, Galion, and Ontario; as well as the villages of Elida and Caledonia.