State Route 502 (SR 502, OH 502) is an east–west state highway in western Ohio, a U.S. state. The western terminus of State Route 502 is at the Indiana State Line approximately 6 miles (9.7 km) south of Union City, with the road continuing into the Hoosier State being locally-maintained Greenville Pike. The eastern terminus of the highway is in downtown Greenville at a traffic circle where it meets a conglomeration of four other state highways: State Route 49, State Route 118, State Route 121 and State Route 571.

State Route 502 was established in the late 1930s. It primarily connects rural areas of west-central Darke County with the county seat of Greenville.

The entirety of State Route 502 is located within Darke County. The state highway is not included as a component of the National Highway System, a network of highways that are determined to be most important for the economy, mobility and defense of the nation.

State Route 502 begins at the Indiana State Line at the State Line Road intersection. The road entering into the intersection from the Indiana side is locally-maintained Greenville Pike. State Route 502 runs due east from the state line into Washington Township, traversing primarily rolling farmlands, with the occasional house and clump of trees along the way. The highway passes Stocksdale Road and Springhill Road, then passes by a more concentrated area of trees prior to intersecting Hillgrove-Southern Road. The highway alternates passing through farmland and a small forest before re-emerging into open farmland as it makes its way to Palestine-Union City Road. For the next two miles, State Route 502 passes some small patches of trees while primarily passing amidst open farm country. In that stretch, the highway intersects Noll Road and then goes on to meet New Madison-Coletown Road, where it crosses from Washington Township into Greenville Township.

Into Greenville Township, the route soon curves to the east-southeast. After passing intersections with Shade Road and Heller Road, the highway meets Greenville-Nashville Road, then bends to the east-northeast. Next, State Route 502 passes Daly Road and Bakers Store-Greenville Road, after which it turns back to the east. The route soon enters into the city of Greenville. Now known as Winchester Avenue, State Route 502 starts to turn southeast as it crosses railroad tracks, then becomes known as Vine Street as it enters into a residential neighborhood. At the Main Street intersection, State Route 502 turns onto Main Street heading northeasterly. It meets Elm Street at a four-way stop intersection, then crosses from a residential area into the central business district. After meeting Sycamore Street at a traffic signal, State Route 502 arrives at its endpoint one block later at a traffic circle where it meets State Route 49, State Route 118, State Route 121 and State Route 571.

The debut of State Route 502 took place in 1937. It was routed along the alignment that it maintains to this day. There have been no changes of major significance to the routing of State Route 502 since its inception.

The entire route is in Darke County.