Kentucky Route 90 originates at a junction with KY 70 near I-65 in Cave City in Barren County. (Signs at the I-65 exit tend to imply that KY 90 ends at the exit itself, while state highway documents indicate that the end is at the intersection with KY 70, about 100 yards (91 m) from the exit proper.) The route continues through Metcalfe County, Cumberland County, Clinton County, Wayne County, Pulaski County and McCreary County to terminate at a junction with U.S. 25W at Youngs Creek in Whitley County nine miles (14 km) northwest of Williamsburg.

At the route's terminus in Whitley County, there is a 0.32 miles (0.51 km) spur called KY 90S that serves as a secondary entrance to the highway for northbound traffic on U.S. 25W. The road is signed as KY 90, however.

KY 90 runs concurrently with U.S. 27 for about 14.6 miles (23.5 km) from Burnside to Parkers Lake. It runs concurrently with U.S. 127 for about a half mile near the community of Snow, and also with KY 61 through the center of Burkesville.

KY 90 is the gateway to Cumberland Falls State Resort Park and (along with US 27) General Burnside State Park.