U.S. Route 12 or US 12 is an east–west United States highway, running from Grays Harbor on the Pacific Ocean, in the state of Washington, to downtown Detroit, for almost 2,500 miles (4,000 km). As a thoroughfare, it has mostly been supplanted by I-90 and I-94, but remains an important road for local and regional travel.

The highway begins in downtown Detroit, at the corner of Michigan and Woodward Avenue. Its western terminus is in Aberdeen, Washington at an intersection with US 101.

The western terminus of US 12 is located in Aberdeen, Washington. In the 1960s, a portion of US 12 was moved north to the town of Morton, when the Mossyrock Dam was built and flooded the towns of Kosmos and Riffe, along the Cowlitz River in Lewis County. A large portion of old, two-lane US 12 was replaced by Interstate 82 and Interstate 182 in the 1980s, between Yakima and the Tri-Cities, though the freeways are still cosigned with the US 12 designation. The old two-lane highway now bears the name Wine Country Road. The highway loosely follows the eastbound leg of the Lewis and Clark Expedition, between Wallula, Washington and Clarkston, Washington, thus being marked as part of the Lewis and Clark National Historic Trail. The east end of the highway in the state is at Clarkston, where the highway crosses the Snake River into Idaho at Lewiston.

The Washington section of US 12, other than a concurrency with Interstate 5, is defined at Washington Revised Code § 47.17.055.

US 12 enters the state at Lewiston, crossing the Snake River from Clarkston, Washington. It ascends the Clearwater River, concurrent with US 95 for 7 miles (11 km) . It reduces to a two-lane undivided highway with signs that read "winding road next 99 miles" and goes on to Orofino, continuing up the middle fork of that river to Lowell, the junction of the Lochsa and Selway Rivers. It continues up the Lochsa and climbs to Lolo Pass at the Montana border. This portion of the highway is also designated as part of the Lewis and Clark National Historic Trail. Most of the highway in Idaho is within the Clearwater National Forest. The eastern section of US 12, through remote mountain forest and up to Lolo Pass, was built in the early 1960s, making US 12 the last US highway constructed. No services are available between Lowell and Powell, about 70 miles (110 km) further east. U.S. Route 12 through Idaho has been proposed as a route for shipment of huge equipment from Lewiston, an inland port, to oil sands facilities near Fort McMurray, Alberta and to a refinery in Billings, Montana. On two-lane portions of the road, the equipment, weighing as much as 300 tons and as much as 30 feet high and 24 feet wide would occupy the entire roadway. The route is preferable to other routes due to the lack of underpasses and the great distances involved. The alternative is transport across the Great Plains from Texas or New Orleans On U.S. 12 the major obstacle would be power lines which would have to be raised or buried. That and other alterations to the highway such as turnouts would be would be paid for by the companies. The trucks would transport only at night, moving short distances between places where they would pull off and let traffic pass. A permit granted by the Idaho Transportation Department to ConocoPhillips in August 2010 is the subject of litigation initiated by householders along the route. On January 19, 2011 it was announced that the Idaho government would issue permits for four loads of refinery equipment to be transported from Lewiston to Billings.

The longest portion of US 12 in any of the states in which it traverses is in Montana. The highway enters Montana at Lolo Pass, 7 miles (11 km) southwest of Lolo Hot Springs, Montana in the Lolo National Forest. After passing Lolo Peak to the south and traveling east for 33 miles (53 km), it meets with US 93 at Lolo and continues as a concurrency northeast for 7.5 miles (12.1 km), where US 93 heads due north on Reserve Street, and US 12 keeps going Northeast to downtown Missoula, eventually meeting Interstate 90. It then overlaps I-90 for 69 miles (111 km), until Garrison, where it heads east through Avon and Elliston. Here US 12 passes through the Helena National Forest and then on to the capital, Helena, where it junctions with Interstate 15 and US 287. It then overlaps US 287 and heads southeast, toward Townsend, where it splits off and heads east until it meets with US 89. It overlaps US 89 for 11 miles (18 km), until just past White Sulphur Springs, where it continues east on its own for 233 miles (375 km), until the junction with Interstate 94 at Forsyth. The major junctions along the way are US 191 at Harlowton and US 87 at Roundup. At exit 93 near Forsyth, US 12 overlaps I-94 for 48 miles (77 km), until Miles City, where it again splits off on its own and heads east for 89 miles (143 km) to southwestern North Dakota, passing through Baker on the way.

US 12 is a two-lane undivided highway that runs 87.47 miles (140.77 km), through Adams, Bowman and Slope counties in southwest North Dakota. The speed limit is 65 mph (105 km/h) on rural segments, with slower posted speeds within the cities of Marmarth, Rhame, Bowman, Scranton and Hettinger. US 12 meets with US 85 in Bowman, and the routes are concurrent for a short distance through the city.

US 12 enters South Dakota from North Dakota, as a rural two lane highway about 10 miles (16 km) west/northwest of Lemmon. For approximately the next 70 miles (110 km), US 12 runs parallel to the border of North Dakota, sometimes within less than a mile. At Walker, US 12 heads southeast for 37 miles (60 km), where it crosses the Missouri River at Mobridge. From there it continues east for 18 miles (29 km), until it meets with US 83 near Selby. It overlaps US 83 for 7 miles (11 km) and for about half of that distance, it is an expressway. After leaving US 83, it turns due east and spends about 80 miles (130 km) as a rural two lane highway again. A few miles before reaching Aberdeen, it becomes an at-grade expressway. After the junction with U.S. Route 281, it goes back to being two lane for a few miles, before once again becoming a 4 lane expressway, until 2 miles (3.2 km) before Waubay. It then meets with Interstate 29 near Summit. From there it heads southeast 22 miles (35 km), until Milbank. At Milbank, it turns back northeast for 10 miles (16 km), until it crosses into Minnesota at Big Stone City, just south of Big Stone Lake. The South Dakota section of US 12 is legally defined at South Dakota Codified Laws § 31-4-132.

From the South Dakota/Minnesota state line at Ortonville, to Wayzata, US 12 is mostly a rural two-lane highway with a 55 mph (89 km/h) speed limit, with slower speed limits through towns and a four-lane surface arterial segment through the city of Willmar. From western Wayzata to Interstate 394 in Minnetonka, US 12 is a six-lane freeway. East of I-494, US 12 is invisibly concurrent with Interstates 394 and 94 through Minneapolis and St. Paul to the Minnesota/Wisconsin state line at Hudson.

The Minnesota section of US 12 is defined as Routes 149, 26, and 10 in Minnesota Statutes §§ 161.115(80) and 161.114(2).

From Elkhorn, Wisconsin to near the Illinois/Wisconsin state line, U.S. 12 is a freeway with a 65 mph (105 km/h) speed limit. It continues as a two-lane highway until it approaches the city of Whitewater, where a new bypass has been constructed. It is currently two lane, but can be expanded to four lanes. U.S. 12 continues west to Madison. As US 12 nears Madison, it merges with US 14, US 151 and US 18, to form the West Beltline Highway, a four to six-lane freeway that encircles the south and west portions of the city, with a speed limit of 55 mph (90 km/h).

From Middleton, the highway continues, on a 15 mile (24 km) four-lane road completed in 2004, to cross the Wisconsin River at Sauk City.

In Illinois, US 12 is an arterial surface road that runs from Richmond, southeast to Des Plaines. It then turns due south, continuing through the Chicago metropolitan area, joining with U.S. Route 45. In Stone Park, U.S. Route 20 joins U.S. 12/45. In Hickory Hills, U.S. 45 continues south, while U.S. 12/20 runs due east along 95th Street in the southwest suburbs. From Hickory Hills, U.S. 12/20 runs east nearly to the Lake Michigan lakefront and then joins with U.S. Route 41, as all three routes travel southeast into the state of Indiana.

US 12 is referred to as Rand Road in Chicago's northwest suburbs. Rand is an original name for the area around Des Plaines, Illinois, the location where the road resumes its westerly direction. South of Des Plaines, U.S. 12 follows Mannheim Road and then 95th Street, before merging with U.S. 41 on Ewing and Indianapolis Avenues toward the Indiana state line.

In Indiana, US 12 is a historically significant route, that winds along the southern coast of Lake Michigan. It runs from an interchange with the Indiana Toll Road, concurrent with US Routes 20 and 41 in Whiting, to Michiana Shores, at the Michigan state line. A large portion of this segment is known as the Dunes Highway.

US 12 is now the only U.S. highway route still serving downtown Detroit, whose street grid was laid by Augustus B. Woodward, to have a five-way intersection of the roads that would become US 12, US 10, US 16, US 112 and US 25. US 24 still travels through Detroit from Puritan to 8 Mile Road on the far-west-side.

As from the earliest days of its existence, US 12 enters Michigan from Indiana, southwest of New Buffalo and continues to the old junction of US 12 and US 112 in New Buffalo. It is now assigned between New Buffalo and Detroit (except through Ypsilanti), along what was US 112 until 1962.

On May 4, 2004, the Michigan Department of Transportation (MDOT) designated 209 miles (336 km) of US-12 from New Buffalo to Detroit as a Historic Heritage Route. The east-west corridor traverses the counties of Berrien, Cass, St. Joseph, Branch, Hillsdale, Lenawee, Washtenaw and Wayne. MDOT's Heritage Route Committee has approved the route's nomination as a premier addition to the Michigan Heritage Route Program.

The U.S. 12 Heritage Trail also opens their doors annually the second weekend of August for what has been dubbed "Michigan's Longest Yard Sale" on Friday, Saturday and Sunday.

Since the highway's creation in 1926, the eastern terminus has always remained within a few blocks of this point.

The western terminus was gradually extended westward, until it met up with the Pacific Ocean.

In 1925, U.S. 12 in Michigan was originally proposed to run from Detroit to Ludington, across Lake Michigan, via the Pere Marquette Railway car ferry to Manitowoc, Wisconsin and then continuing into Wisconsin, on what later became US-10, in those two states.

U.S. 12 originally went into Wyoming, before being rerouted into Montana and was proposed to go into Oregon, but did not.

U.S. 12 was originally planned to be a freeway from Chicago to Madison, Wisconsin. The portion from Genoa City, WI, to Elkhorn, WI was already being built when the Interstate system was announced. After the Interstate program was announced and due to the fact that Wisconsin was not able to obtain some right-of-way land north of Elkhorn, construction was stopped. Since construction was stopped, Illinois never built any of it, but the rough grading and off-ramps are still there in Illinois. In Elkhorn, the ramps also continue past the intersection where the construction stopped, but they are not used. Much of U.S. 12 in Illinois is a divided highway and it is surface road.