PA 366 begins at an interchange with the PA 28 freeway in Fawn Township, Allegheny County, heading south on Bull Creek Road, a four-lane divided highway that is part of the Red Belt of the Allegheny County belt system.

[1][2] Upon crossing the river, the route enters the city of New Kensington in Westmoreland County, becoming four-lane undivided Tarentum Bridge Road, passing between residential neighborhoods to the west and businesses to the east.

The route continues southeast through more woodland with homes and businesses, crossing the Pucketa Creek into the borough of Plum in Allegheny County.

PA 366 continues past more wooded residential areas, crossing into the borough of Murrysville in Westmoreland County.

The route heads to the east as it enters a mix of farm fields and woods with a few residences, ending at an intersection with PA 66.