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.

Ohio 309's current route was the route of U.S. Route 30S until that split was decertified in 1975. Under Ohio numbering custom, the 400 series of numbers was traditionally used to renumber decertified and remnant highways. However, State Route 430 had already been certified from Mansfield to Mifflin as a remnant of US 30. Despite the fact that Ohio 430 ended on US 30S, it was decided that US 30S would be recertified with a new number that looked similar to 30S, so it would not confuse travelers of the route.[citation needed] The route was thus certified as SR 309.

The only significant change of the route since this certification is its western terminus, which originally was with U.S. 30 in Delphos. When U.S. 30's bypass around Delphos was completed in 1981, Ohio 309's western terminus was made the easternmost interchange of the bypass.

Before being used as its current route, the route number 309 was used as the route number of what is now State Route 541. It was replaced with the now defunct State Route 271 in 1936.

The Lincoln Highway left the present State Route 309, a better-quality road at the time, in favor of the direct Delphos-Mansfield route now generally followed by US 30. Only two weeks after the official route was announced in September 1913, it had already been redefined to a straighter path between Lima and Galion; the rest of the straightening came later. The towns along the route got together and formed the Harding Highway, named after President Warren G. Harding, who had grown up in the area.

This Harding Highway continued east via the Lincoln Highway to Canton, turning southeast from there to Steubenville via State Route 43. To the west, it was marked along the Lincoln Highway to Van Wert, heading west along the present U.S. Route 224, roughly State Road 114, State Road 10, State Road 8, and State Road 2 through Indiana to the Illinois state line.