State Route 703 (SR 703) is a state highway in western Ohio. Its western terminus is in Montezuma at State Route 219, and its eastern terminus at an intersection with State Route 29, State Route 66, and the southern terminus of State Route 116 in St. Mary's. The route primarily runs along the western and northern shores of Grand Lake St. Marys, causing the route to run in more of a north-south fashion toward its western end between Celina and Montezuma. However, the route is primarily an east-west highway.

SR 703 travels through eastern Mercer County and the westernmost portion of Auglaize County. No portion of this highway is included within the National Highway System (NHS). The NHS is a network of routes deemed to be most important for the economy, mobility and defense of the nation.