State Route 604 (SR 604, OH 604) is an east–west state highway in north central Ohio. The western terminus of State Route 604 is at a T-intersection with U.S. Route 42 approximately 5 miles (8.0 km) southwest of West Salem. Its eastern terminus is at the intersection that doubles as the southern split of the duplex of State Route 94 and State Route 585 nearly 1.5 miles (2.4 km) southeast of Rittman.

Created in the late 1930s, this primarily rural route traverses the northeastern corner of Ashland County and the northern tier of Wayne County. State Route 604 passes through one incorporated community along its path, Congress, where it meets State Route 539.

State Route 604 travels through the counties of Ashland and Wayne. No portion of this state highway is included within the National Highway System, a network of routes deemed most important to the country's economy, mobility and defense.

State Route 604 was first designated in 1937 along the path through northeastern Ashland County and northern Wayne County that it occupies to this day. The highway has not experienced any major changes since making its debut.