State Route 635 (SR 635, OH 635) is a north–south state highway in the northwestern portion of the U.S. state of Ohio. The southern terminus of State Route 635 is at State Route 18 in Bascom. Its northern terminus is approximately 1 mile (1.6 km) east of Helena at a T-intersection with U.S. Route 6.

State Route 635 travels through northwestern Seneca County and southwestern Sandusky County along its way. The highway is not included as a part of the National Highway System, a network of highways deemed to be most important for the nation's economy, mobility and defense.

The debut of State Route 635 took place in 1937. Its original routing between State Route 18 in Bascom and U.S. Route 6 near Helena is the same one that it utilizes to this day. No significant changes have taken place to the alignment of State Route 635 since it was established.