U.S. Route 12, (US 12), is a major east–west U.S. Highway, running from Aberdeen, Washington, to Detroit, Michigan. In the state of Washington, it runs for 403.80 miles (649.85 km), and is the only numbered highway to span the entire state from west to east, starting near the Pacific Ocean, and crossing the Idaho state line near Clarkston. It crosses the Cascade Range over White Pass, south of Mount Rainier National Park. Portions of it are concurrent with Interstate 5 (I-5) and Interstate 82 (I-82), although the majority of the route does not parallel any Interstate highway.

Although US 12 was not extended into Washington until 1967, portions of it have been part of Washington's state highway system since as early as 1905. The last part of the highway to open was over White Pass in 1951, although it was added to the state highway system by the legislature in 1931. Most of the route (except for the approximately 160 miles (260 km) between Elma and Naches) had been part of the U.S. Highway System since its inception in 1926 as part of US 410. The portion between Napavine and Grand Mound was also designated a U.S. Highway in 1926 as part of US 99.

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