State Route 191 (abbreviated as SR 191 or OH 191) is a north–south state highway in northwestern Ohio, a U.S. state. State Route 191's southern terminus is at U.S. Route 6 approximately 4 miles (6.4 km) south of Stryker. Its northern terminus is at the duplex of U.S. Route 20A and U.S. Route 127 in downtown West Unity. Between its southern terminus and Stryker, State Route 191 runs parallel to, and to the east of, the Tiffin River.

State Route 191 begins at an intersection with US 6 and begins north toward Stryker. Just north of the southern terminus, SR 191 curves northwest, then reaches an intersection with SR 2 and SR 34. At this intersection, SR 2 becomes concurrent with SR 191 and the routes continue north until reaching the city of Stryker. Entering Stryker on Defiance Street, SR 191 forks west onto Lynn Street, while SR 2 continues north on Defiance Street. Outside of Stryker, the route curves north. After a few miles, SR 191 briefly turns west, then turns north and heads toward West Unity. Inside the city limits of West Unity, SR 191 is known as Main Street. The route ends at its northern terminus at an intersection with US 20 and US 127 in West Unity.

All of State Route 191 exists within the confines of Williams County. No segment of this route is incorporated within the National Highway System, a system of highways considered important to the nation's defense, economy, and mobility.

At its inception in 1924, State Route 191 consisted of the following route: Defiance County Road 1 from the point where State Route 66 crosses the boundary between Defiance and Williams Counties west to Evansport; Williams County Road 22-A from Evansport northwest to the current routing of U.S. Route 6 at the current southern terminus of State Route 191; and the current portion of State Route 191 from there to State Route 2 in Stryker. In 1937, the highway was extended north from Stryker to its current northern terminus at the duplex of U.S. Route 20A and U.S. Route 127 in West Unity. Then, in 1962, the highway took on its present form when the portion of State Route 191 south and east of the existing U.S. Route 6 was removed from the state highway system, with the segment from the current southern terminus to Evansport renamed as Williams County Road 22-A, and the part from Evansport east that, for the most part, straddles the Defiance-Williams County Line given the name of Defiance County Road 1.

The entire route is in Williams County.