Hudson to Lake Delton

US 12 crosses the St. Croix River from Minnesota concurrent with I-94 into St. Croix County. Wisconsin Highway 35 (WIS 35) joins both routes upon their entry into Wisconsin and exits south three miles (4.8 km) east. US 12 leaves I-94 one mile (1.6 km) further east from the eastern WIS 35 exit, treks two miles (3.2 km) north, and parallels the Interstate. The highway crosses WIS 65 in Roberts and passes through Hammond over a 15-mile (24 km) segment to Baldwin, where it crosses US 63. US 12 passes through Woodville four miles (6.4 km) east of Baldwin and crosses WIS 128 in Wilson, two miles (3.2 km) west of the Dunn County line.^[2] Roughly three miles (4.8 km) into Dunn County, US 12 passes through Knapp then turns southeastward, connecting with WIS 79 six miles (9.7 km) further; it then joins WIS 25 (North Broadway Street) to the south in Menomonie. US 12 turns east onto WIS 29 (Main Street East) in downtown Menomonie, and the two routes follow Stout Road east out of the city.^[3] The highways meet an interchange with I-94, then split one half of a mile (0.80 km) east in Elk Mound at the junction with WIS 40. US 12 passes through Elk Mound and enters Eau Claire County.^[4]

US 12 merges with WIS 312 east (North Crossing) for two miles (3.2 km) before turning south onto Clairemont Avenue, bypassing downtown Eau Claire to the south and west. WIS 37 joins with US 12 on the southwest side of the city. Access to WIS 93 is provided via US 53. The interchange between US 12 and US 53 in Altoona is Wisconsin's first single-point urban interchange.^[5] US 12 continues east out of the Eau Claire–Chippewa Falls metropolitan area and passes through Fall Creek, where it turns southeastward to join WIS 27 south in Augusta. The two highways briefly join US 10 on the Jackson County line in Fairchild.^[6] US 12 and WIS 27 turn southward and pass through Humbird in Clark County and into Jackson County.^[7]

US 12 and WIS 27 cross WIS 95 in Merrillan and WIS 54 in Black River Falls. US 12 and WIS 27 also split in Black River Falls. US 12 follows I-94 to the southeast past Millston and into Monroe County.^[8] The highway crosses I-94 (with no access) south of Kirby and junctions with WIS 21 on the north side of Tomah at an interchange with the Interstate. US 12 passes through Tomah and joins with WIS 16 on the south side at the junction with WIS 131 near Mill Bluff State Park. The highways continue paralleling I-90 and I-94 southeast and pass through Oakdale and into Juneau County.^[8]

US 12 and WIS 16 pass through Camp Douglas and cross WIS 80 in New Lisbon. The highways then junction with WIS 58 and WIS 82 in Mauston as they continue southeast. The routes trek eastward and pass through Lyndon Station, where Rocky Arbor State Park is located. The highways turn southeast again and cross the Interstates into Sauk County and Wisconsin Dells.^[9] WIS 16 turns east onto WIS 13 north as WIS 23 west turns south onto US 12. These highways follow the Wisconsin Dells Parkway south into the heart of the Wisconsin Dells tourism district,^[10] passing such attractions as Noah's Ark Water Park,^[11] the Wisconsin Ducks boat tours,^[12] Kalahari Resort, Mt. Olympus Water & Theme Park, Wilderness Territory, and Tommy Bartlett Show.^[13] WIS 23 turns west onto Monroe Avenue as US 12 turns southeast and crosses the Interstates.^[14]

Lake Delton to Cambridge

At the I-90/94 interchange, US 12 becomes a freeway that bypasses Baraboo that goes just south of the Grasser Road overpass. The highway becomes an expressway just north of Lehman Road and remains so until a few miles north of Sauk City. WIS 60 briefly joins US 12 in Sauk City and WIS 78 crosses the Wisconsin River concurrently with US 12 into Dane County.^[14] US 12 becomes an expressway again at the county line and continues southeast, crossing WIS 19 at Springfield Corners. Just northwest of Middleton, the route becomes a freeway as it enters the Madison area along the West Beltline Highway.^[15]

US 12 merges with US 14 at University Avenue. The freeway continues south and passes the interchanges of Greenway Blvd, Old Sauk Road, and Mineral Point Road (CTH-S). The freeway then turns eastward and passes the interchanges of Gammon Road, Whitney Way, and Verona Road/Midvale Blvd, where US 151 and US 18 join the freeway, creating a concurrency of four US Highways for a few miles. During the four-way concurrency, the freeway passes the interchange of Seminole Highway (only accessible from the westbound side), the University of Wisconsin–Madison Arboretum, and the interchanges of Todd Drive, Fish Hatchery Road (CTH-D), and Park Street, where US 14 leaves to go south and US 151 north into downtown Madison. The freeway had several ramps, which contained driveways that accessed businesses.^[16] US 12 and US 18 continue east into Monona and cross the Upper Mud Lake channel. The highways meet US 51 at Stoughton Road, then interchange with I-39 and I-90 on the southeast side of Madison.^[17] US 12 and US 18 continue as an expressway east of the Interstates, junctioning with WIS 73 in Deerfield and splitting, with US 12 turning southeast into Jefferson County in Cambridge.^[15]

Cambridge to Genoa City

After leaving Cambridge, US 12 passes through Oakland and crosses WIS 26, WIS 89, and WIS 106 in Fort Atkinson. WIS 89 joins US 12 as the route turns southward to approach Whitewater, bypassing the city to the south into Walworth County. The route briefly enters Rock County before crossing into Walworth County.^[18] WIS 89 turns off US 12 at its junction with WIS 59. WIS 59 East follows US 12 briefly before turning north into the city. US 12 passes through La Grange and turns south at the junction with WIS 20 to follow WIS 67 south. US 12 turns southeast off WIS 67 onto a freeway northeast of Elkhorn. The highway crosses over WIS 11 with no access (although access is provided via nearby I-43), then interchanges with I-43. Access for Lake Geneva is provided at WIS 120 and WIS 50 as US 12 passes the city to the east. The highway turns south into Genoa City, where the freeway ends and US 12 follows the crossroad into Illinois.