US Route 12 (US 12) is a federal highway in north central Idaho. It extends 174.210 miles (280.364 km) from the Washington state line in Lewiston east to the Montana state line at Lolo Pass, generally along the route of the Lewis and Clark expedition, and is known as the Northwest Passage Scenic Byway It was previously known as the Lewis and Clark Highway.

US 12 enters Idaho at the Washington state line in Lewiston, Nez Perce County, crossing the Snake River at the state line. It heads east through Lewiston, turning north to cross the Clearwater River and intersect SH-128. It continues east to overlap US 95 along a limited access section. The overlapped highways run east along the north bank of the Clearwater River for 7.299 miles, leaving Lewiston and entering the Nez Perce Indian Reservation before separating.

US 12 then continues east along the north bank of the Clearwater through North Lapwai, past the Ant and Yellowjacket rock formation and a historical marker for the Spalding Mission. It continues east to intersect SH-3 and cross the Clearwater again.

US 12 then continues east along the south bank of the Clearwater past historical markers commemorating Indian houses and the ghost town of Slaterville, and, in a rest area at Lenore, a historical marker for the Lenore Tram. It then continues into Clearwater County.

In Clearwater County, US 12 continues east along the south bank of the Clearwater through Orofino. Just after leaving Orofino, it briefly overlaps SH-7, then continues southeast along the south bank of the Clearwater past a historical marker for the point where Lewis and Clark first found a western-flowing river, and into Lewis County. It then continues into Lewis County.

In Lewis County, US 12 continues southeast along the south bank of the Clearwater, intersecting SH-11 at Greer and passing a historical marker for a ferry operated in the 1860 gold rush. It then continues southeast into Kamiah, where it intersects SH-162. It then crosses the Clearwater again, leaving Kamiah and entering Idaho County.

In Idaho County, US 12 continues south along the north bank of the Clearwater, crossing Nez Perce National Historical Park. In the park, it passes a historical marker for two sites located about two miles away, commemorating the Lewis and Clark Long Camp of 1806 and the Asa Smith mission of 1839 to 1841. It then continues south along the north bank of the Clearwater, intersecting SH-13 across the river from Kooskia. It then turns east along the north bank of the Clearwater, passing a historical marker commemorating the camp of Nez Perce led by Looking Glass, and the July 1, 1877 attack by the U.S. Army that provoked Looking Glass to join the Nez Perce retreat with Chief Joseph. It continues east along the north bank of the Clearwater, leaving the Nez Perce reservation. US 12 then continues to Lowell, where it turns northeast along the north bank of the Lochsa River through the Rocky Mountains.

In the Rockies, US 12 passes historical markers for Whitehouse Pond, Lewis and Clark's crossing of the Lolo Trail in 1806, and their crossing of the Lolo Pass summit in 1805, before crossing Lolo Pass at the Montana state line.

US 12 was created in 1925 as part of the original system of United States highways. Its original western terminus was in Miles City, Montana. In 1962, the highway was extended west to Lewiston, ending at the former US 410. In 1967, it was extended to its current western terminus in Aberdeen, Washington, with the Idaho section taking its current route.

The Lewis and Clark Highway, from Lewiston eastward to Lolo Pass, was designated State Route 9 in 1916 and construction began in 1920. Federal prison labor was used in the late 1930s and Japanese internment labor was used during World War II, working out of the camp six miles (10 km) upstream of Lowell. By the fall of 1955, 27 miles (43 km) remained unfinished. Upon its completion in 1962, it was redesignated US 12.

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 Port of Lewiston is the furthest inland seaport on the West Coast of the United States of America. It ships wheat, barley, legumes to Asia and the South Pacific as well as the Middle East and Africa.There are also inland seaports at the port of Clarkston, Washington and Port of Wilma in Whitman County, WA.