State Route 18 is an east–west highway in northern Ohio. Its western terminus is at the Indiana state line near Hicksville, Ohio (the route continues in Indiana as State Road 8), and its eastern terminus is at State Route 91 in Akron, Ohio.

State Route 18 was an original state highway that went from Norwalk to the Pennsylvania state line. The route was extended to the Indiana state line in 1926. Until 1950, it was one of a very few Ohio routes to end at two state lines.

State Route 18's extension to the Indiana state line originally overlapped State Route 2 from the line to Hicksville. In 1940, State Route 18 was rerouted on the former State Route 193 from the line to Hicksville.

In 1950, State Route 18's eastern terminus was moved to Youngstown. Its old route to the Pennsylvania state line was recertified as State Route 289.

In 1966, the route was routed along State Route 8 and Interstate 80S (now Interstate 76) to Youngstown. This series of concurrencies ended in 1971, and State Route 18 would eventually be routed along Market Street in Akron, to end at its current terminus.

In 1969, State Route 18 was routed concurrently with U.S. Route 20 along the Norwalk Bypass, a limited access freeway. Prior to the Bypass route, State Route 18 followed Main Street and Woodlawn Avenue (formerly known as Medina Street) and later followed Main Street and Akron Road through the city of Norwalk (much of this alignment was already concurrent with US 20).

Before the rerouting of SR 18 via SR 8 and I-80S, SR 18 left West Market Street at Twin Oaks Drive in West Akron, where it went a few blocks east to North Portage Path, then jogged 1 block north to Memorial Parkway, which became Tallmadge Avenue east of the Cuyahoga River (before that, SR 18 continued along Market Street, then joined with SR 8 before reaching Tallmadge Avenue).

Tallmadge Road was a straight east/west road which became Mahoning Avenue as it approached the Youngstown area.

Most stretches of former SR 18 are still referred to as County Road 18 in Portage and Mahoning Counties.