Kentucky Route 139 (KY 139) is a 55.797-mile-long (89.797 km) state highway in Kentucky that runs from Tennessee State Route 120 at the Tennessee state line south of Cadiz to Kentucky Route 120 in rural Crittenden County north of Princeton via Cadiz and Princeton.

KY 120 begins at the Tennessee state line just west of the Fort Campbell Military Reservation.

After entering Caldwell County, it then intersects Interstate 24 (I-24) where north of the ramp is the frontage roads, KY 6059 and KY 6060.

KY 91/KY 139 together turns right on the west side of town and then intersects with Interstate 69 (I-69).

Then KY 139 ends at a junction with KY 120 in rural southeastern Crittenden County just west of the Tradewater River.