State Route 661 (SR 661) is a 22.22-mile (35.76 km) long north–south state highway in the central portion of the U.S. state of Ohio. The southern terminus of SR 661 is at a diamond interchange with SR 16 and SR 37 just 0.25 miles (0.40 km) south of Granville. SR 661 has its northern terminus at a signalized T-intersection with SR 13 in the southern end of Mount Vernon.