Beloit to Stevens Point

US 51 enters Wisconsin in Beloit on Broad Street as a four-lane divided highway but immediately merges into a two-lane urban road. The route turns right onto Pleasant Street and begins running along the Rock River. The road passes Beloit College and crosses Wisconsin Highway 81 (WIS 81) before leaving Beloit and going toward Janesville. Along the way, it goes under the Wisconsin and Southern railroad and turns away from the river and passes Southern Wisconsin Regional Airport. US 51 becomes a four-lane undivided highway as it crosses WIS 11.^[2]

As US 51 enters Janesville, it crosses the Rock River and runs on Center Street. The road turns northeast, running parallel to the WSOR railroad before crossing the river again and turning onto North Parker Drive. US 51 passes under the railroad and temporarily becomes a four-lane divided highway as it parallels the Rock River again. US 51 leaves Janesville as a two-lane road and crosses US 14 as it makes its way to Edgerton. Along the way it crosses the Rock River. US 51 passes through Edgerton. A few miles north of Edgerton, joins with I-39/I-90 for about five miles (8.0 km) before splitting off and heading west to Stoughton. In Stoughton, US 51 turns north and passes Lake Kegonsa and goes through McFarland, where it becomes an expressway. The expressway enters Madison and crosses the Beltline (US 12/US 18) and continues through the east side of Madison as a side route, known locally as Stoughton Road, paralleling to the west of I-39/I-90. The expressway crosses WIS 30 and US 151, where it downgrades to an urban four-lane road. Between WIS 30 and US 151, the highway is crossed by a railroad track at the surface. US 51 passes Madison Area Technical College and Dane County Regional Airport before going under the Interstate.^[2]

North of Madison, US 51 parallels the Interstate on the east side passing through De Forest, Leeds, North Leeds, Poynette and Portage. In North Leeds, US 51 intersects WIS 22 and WIS 60 at a roundabout; US 51 heads west a bit with WIS 60 before splitting back north toward Poynette. The route joins I-39 north of Portage. There is a short concurrency with WIS 23 from Endeavor to WIS 82 near Oxford. US 51 stays joined with I-39 until the Interstate ends at WIS 29. Along the way, the route bypasses the small communities of Endeavor, Packwaukee, Westfield, Coloma, Hancock, Plainfield, Plover, and Whiting prior to arriving at Stevens Point. In Stevens Point, the route runs concurrently with US 10.^[2]

Stevens Point to Merrill

Proceeding north from Stevens Point, the highway bypasses Knowlton, Mosinee, and Rothschild where I-39 terminates at the WIS 29 east interchange. Along the way, US 10 turns to go west toward Minneapolis. US 51 continues on with WIS 29 for five miles (8.0 km) bypassing Schofield, then proceeding into Wausau where WIS 29 splits to the west.^[2]

After Wausau, US 51 continues as a freeway to Merrill. Along the way, it passes Brokaw. For much of this stretch, the highway follows the Wisconsin River. The highway passes an exit with County Trunk Highway Q (CTH-Q) just before crossing the Wisconsin River. It then passes Merrill, with exits at WIS 64 and CTH-K.^[2]

Merrill to Hurley

A few miles north of Merrill, the freeway downgrades to an expressway for nine miles (14 km). Within the expressway exists six grade intersections at Lincoln Drive, CTH-K (Nelson Avenue), CTH-J, CTH-V, a private road, Tamarack Lane; as well as several private driveways. The highway resumes as a freeway and passes three Tomahawk exits, CTH-S. WIS 86/CTH-D, and CTH-A. Between WIS 86 and CTH-A, the highway crosses the Wisconsin River. Just before the freeway ends again it passes an exit with US 8.^[2]

Just after the US 8 exit, at the Oneida County line, the freeway ends again and merges into a two-lane rural road. The highway passes through Hazelhurst and becomes a four-lane undivided highway. The highway then passes through Minocqua and Woodruff. WIS 70 runs concurrently with US 51 for a few miles in Woodruff.^[2] For much of this stretch the highway runs parallel to the old Milwaukee Road right-of-way, now the Bear Skin State Trail.^{[citation needed]}

North of Woodruff, the highway becomes a two-lane road again. North of Woodruff, the highway turns northwest and passes through Manitowish Waters, and Mercer. Shortly after Mercer, the highway turns to go straight north and goes through Hurley. Just after Hurley, US 51 ends at a roundabout with US 2 just a couple hundred feet west of the Michigan border.^[2]