U.S. Route 20

U.S. Route 20 or U.S. Highway 20 (US 20) is an east–west United States Numbered Highway that stretches from the Pacific Northwest east to New England. The "0" in its route number indicates that US 20 is a major coast-to-coast route. Spanning 3,365 miles (5,415 km), it is the longest road in the United States, and, in the east, the route is roughly parallel to Interstate 90 (I-90), which is the longest Interstate Highway in the U.S. There is a discontinuity in the official designation of US 20 through Yellowstone National Park, with unnumbered roads used to traverse the park.

US 20 and US 30 break the general U.S. Route numbering rules in Oregon, since US 30 actually starts north of US 20 in Astoria, and runs parallel to the north throughout the state (the Columbia River and Interstate 84). The two run concurrently and continue in the correct positioning near Caldwell, Idaho. This is because US 20 was not a planned coast-to-coast route while US 30 was. US 20 originally ended at the eastern entrance of Yellowstone Park; it was extended in 1940.

The highway's eastern terminus is in Boston, Massachusetts, at Kenmore Square, where it meets Massachusetts Route 2. Its western terminus is in Newport, Oregon, at an intersection with US 101, within a mile of the Pacific Ocean.