SR 366 is a 28.33-mile (45.59 km) highway that connects Mount Graham with US 191 (originally US 666) at Swift Trail Junction south of Safford.

The western terminus of the highway is located near a ranger station near the peak of Mount Graham.

The terrain eventually smooths out and the highway follows a straight path towards the northeast to its eastern terminus at US 191.

SR 366 traverses sparsely inhabited forest and mountain terrain and does not pass through any cities or towns aside from minor settlements.

The highway does provide access to Mount Graham, one of the higher peaks in Arizona at over 10,000 feet (3,000 m).