State Route 366 (SR 366, OH 366) is an east–west state highway in the western portion of the U.S. state of Ohio. State Route 366 has its western terminus at a T-intersection with U.S. Route 33 nearly 1.50 miles (2.41 km) northwest of Lakeview. The route's eastern terminus is at State Route 117 about 1.50 miles (2.41 km) north of Huntsville.

The entirety of State Route 366 is located within the northwestern quadrant of Logan County. This state highway is not a part of the National Highway System, a network of routes deemed to be most important for the economy, mobility and defense of the nation.

The State Route 366 designation was assigned in 1934. The original routing of State Route 366 consisted only of the current stretch of the highway between what was originally designated State Route 32 (now U.S. Route 33) and what was then designated as State Route 69 (now State Route 235) north of Lakeview. At that time, the remainder of the current State Route 366 consisted of State Route 69 and U.S. Route 33. By 1961, a new two-lane alignment of U.S. Route 33 was established through Lakeview, Russells Point and to the southeast. Consequently, State Route 366 took on the shape that it has today by being extended via a concurrency with State Route 69 south into Lakeview, and the former routing of U.S. Route 33 along the south shore of Indian Lake from that point east to its current eastern terminus at State Route 117.

The entire route is in Logan County.