In the U.S. state of Wisconsin, U.S. Route 12 (normally called U.S. Highway 12, Highway 12 or U.S. 12) runs east–west across the western to southeast portions of the state. It enters from Minnesota (concurrently with Interstate 94) at Hudson, parallels the interstate to Wisconsin Dells, and provides local access to cities such as Menomonie, Eau Claire, Black River Falls, Tomah and Mauston. It then provides an alternative route for traffic between northwestern Wisconsin and Madison and is the anchor route for the West Beltline Highway around Madison. Finally, it serves southeastern Wisconsin, connecting Madison with Whitewater, Elkhorn and Lake Geneva. The West Beltline Highway and the segment between Elkhorn and Lake Geneva are freeways, and the segment between Sauk City and Middleton is an expressway. The remainder of the road is a two-lane surface road or an urban multi-lane arterial. Between Hudson and west of Warrens, the road closely parallels the former main line of the Omaha Road, now operated by Union Pacific Railroad.

U.S. 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 (5 km) east. U.S. 12 leaves I-94 one mile (1.6 km) further east from the eastern WIS 35 exit, treks two miles (3 km) north and parallels the interstate. The highway crosses WIS 65 in Roberts and passes through Hammond over a fifteen-mile (24 km) segment to Baldwin, where it crosses U.S. 63. U.S. 12 passes through Woodville four miles (6 km) east of Baldwin and crosses WIS 128 in Wilson, two miles (3 km) west of the Dunn County line. Roughly three miles into Dunn County, U.S. 12 passes through Knapp, Wisconsin, then turns southeastward, connecting with WIS 79 six miles (10 km) further; it then joins WIS 25 (North Broadway Street) to the south in Menomonie. U.S. 12 turns east onto WIS 29 (Main Street East) in downtown Menomonie, and the two routes follow Stout Road east out of the city. The highways meet an interchange with I-94, then split one half of a mile east in Elk Mound at the junction with WIS 40. U.S. 12 passes through Elk Mound and enters Eau Claire County

U.S. 12 merges with WIS 312 east (North Crossing) for two miles (3 km) before turning south onto Clairemont Avenue, bypassing downtown Eau Claire to the south and west. WIS 37 joins with U.S. 12 on the southwest side of the city. Access to WIS 93 is provided via U.S. 53. The interchange between U.S. 12 and U.S. 53 in Altoona is Wisconsin's only single-point urban interchange. U.S. 12 continues east out of the Eau Claire metropolitan area and passes through Fall Creek, where it turns southeastward to join WIS 27 south in Augusta. The two highways briefly join U.S. 10 on the Jackson County line in Fairchild U.S. 12 and WIS 27 turn southward and pass through Humbird in Clark County and into Jackson County.

U.S. 12 and WIS 27 cross WIS 95 in Merrillan and WIS 54 in Black River Falls. U.S. 12 and WIS 27 also split in Black River Falls. U.S. 12 follows I-94 to the southeast past Millston and into Monroe County. 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. U.S. 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.

U.S. 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. WIS 16 turns east onto WIS 13 north as WIS 23 west turns south onto U.S. 12. These highways follow the Wisconsin Dells Parkway south into the heart of the Wisconsin Dells tourism district, — passing such attractions as Noah's Ark Waterpark, the Wisconsin Ducks boat tours, Kalahari Resort, Mt. Olympus Water & Theme Park, Wilderness Territory, and Tommy Bartlett's Thrill Show. WIS 23 turns west onto Monroe Ave as U.S. 12 turns southeast and crosses the Interstates.

At the I-90/94 interchange, U.S. 12 becomes a freeway that bypasses Baraboo. The section from I-90/94 to just south of Old Highway 33 is complete, and then U.S 12 joins it's old alignment at a roundabout. WIS 33 joins the old U.S. 12 alignment three miles (5 km) north of Baraboo; the routes split in West Baraboo at WIS 136. U.S. 12 joins with WIS 159, a short highway that provides access to Devil's Lake State Park. Eventually, WisDOT will continue the Baraboo bypass freeway south of it's current terminus at Old Highway 33, to just south of the Grasser Road overpass. Near the U.S. 12/WI 159 intersection, the highway will become an expressway to just south of Ski Hi Road. The highway briefly becomes an expressway two miles south of WIS 159 for seven miles (11 km) as it approaches Sauk City. WIS 60 briefly joins U.S. 12 in Sauk City and WIS 78 crosses the Wisconsin River concurrently with U.S. 12 into Dane County. U.S. 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.

U.S. 12 merges with U.S. 14 at University Avenue. The freeway continues south and turns eastward at the interchange with CTH M and CTH S (Mineral Point Road). U.S. 18 and U.S. 151 join the beltline off Verona Road in southwestern Madison. The four routes travel concurrently for two miles (3 km), passing through interchanges with Todd Drive and Fish Hatchery Road before U.S. 14 leaves south towards Janesville and U.S. 151 north into downtown Madison via Park Street. The four-way concurrency was crossed by a railroad track at the surface until 2007. The crossing was isolated using Jersey barriers until that time. The freeway also had several ramps, which contained driveways that accessed businesses. U.S. 12 and U.S. 18 continue east into Monona and cross the Upper Mud Lake channel. The highways meet U.S. 51 at Stoughton Road, then interchange with I-39 and I-90 on the southeast side of Madison. U.S. 12 and U.S. 18 continue as an expressway east of the interstates, junctioning with WIS 73 in Deerfield and splitting, with U.S. 12 turning southeast into Jefferson County in Cambridge.

After leaving Cambridge, U.S. 12 passes through Oakland and crosses WIS 26, WIS 89, and WIS 106 in Fort Atkinson. WIS 89 joins U.S. 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 WIS 89 turns off U.S. 12 at its junction with WIS 59. WIS 59 East follows U.S. 12 briefly before turning north into the city. U.S. 12 passes through La Grange and turns south at the junction with WIS 20 to follow WIS 67 south. U.S. 12 turns southeast off WIS 67 onto a freeway northeast of Elkhorn. The highway crosses over WIS 11 with no access, then interchanges with I-43. Access for Lake Geneva is provided at WIS 120 and WIS 50 as U.S. 12 passes the city to the east. The highway turns south into Genoa City, where the freeway ends and U.S. 12 follows the crossroad into Illinois.

The entire route was originally signed as WIS 12 in 1917 prior to the creation of the U.S. highway system in 1926. Aside from changes resulting from the construction of freeways and U.S. 12 being aligned on them, some differences exist between the original route and today's alignment. WIS 12 followed CTH E, a more southerly alignment, between Menomonie and Eau Claire. From Black River Falls, WIS 12 continued south to Shamrock and turned east to follow CTH O to Millston. WIS 12 ran southerly from Tomah along WIS 131 to CTH A and turned east onto the county road to pass through Hustler and Clifton to reach New Lisbon.

The route between Cambridge and Whitewater was upgraded to a straighter and flatter path for safety. A bypass of Whitewater was added in 2005, although the bypass is currently only two lanes. Upgrades to U.S. 12 between Middleton and Sauk City were completed in 2005, but local opposition prevented most of the remaining route north of that point to Lake Delton from being constructed.

In Wisconsin, the highway was designated as the Iron Brigade Memorial Highway in 1993 to honor the Civil War Union Army unit; it also has this designation in Michigan, Indiana and Illinois.

The state originally planned to upgrade all of U.S. 12 from Madison to Genoa City to freeway in anticipation of Illinois facilitating a toll road south of the state line. However, after Wisconsin started upgrading between Elkhorn and Genoa City, it was realized that the project was shelved by Illinois, apparently due to community opposition (see FAP 342 and FAP 420).

Nonetheless, several parts of U.S. 12 between Genoa City and Madison are being studied for possibly upgrade to a freeway. A small, 3.5 mile section from the I-39/90 interchange to County N interchange east of Madison is being complicated as part of the I-39/90 reconstruction project. An extension of the U.S 12 freeway north of Elkhorn to Whitewater is currently being proposed as well. When that happens, the Whitewater bypass will be expanded to a four-lane freeway. A connection between the Whitewater and Fort Atkinson bypass's was in the planning stages, but that has been delayed until 2019 at the earliest.

When all of those projects are constructed, only the section between Fort Atkinson, WI and the County N interchange will need to be upgraded to a freeway. As of 2013, there are no plans for any expansion projects between Fort Atkinson and County N.

(Section needs updating to add exit numbers for new Baraboo Bypass)