Interstate 39 (I-39) is a highway in the Midwestern United States. I-39 runs from Normal, Illinois, at I-55 to State Trunk Highway 29 (WIS 29) in the town of Rib Mountain, Wisconsin, which is approximately six miles (9.7 km) south of Wausau. I-39 was designed to replace U.S. Route 51 (US 51), which, in the early 1980s, was one of the busiest two-lane highways in the United States. I-39 was built in the 1980s and 1990s.

U.S. Highway 51 (US 51) in the U.S. state of Wisconsin runs north–south through the central part of the state. It enters from Illinois at Beloit, and runs north to its northern terminus in Hurley at a roundabout junction with US 2. Some of the route of US 51 runs concurrently with Interstate 39 (I-39).