Ohio State Route 81 runs east-west along a border between northwestern Ohio and west-central Ohio. It parallels US Route 30 by several miles to the south, and it runs through downtown Lima. The route begins as Indiana State Route 124 crosses the state line into Ohio, and it immediately reaches the border town of Willshire, Ohio, where it meets US Route 33. Ohio Route 81 crosses southern Van Wert County, bound for Lima, in Allen County. The highway serves as one of the main east-west thoroughfares in Lima before continuing east toward Ada. After passing through Ada, Ohio Route 81 crosses US Route 68 at Dunkirk, and then ends at Ohio Route 53 in the tiny town of Patterson, in northeastern Hardin County.