U.S. Route 1 or U.S. Highway 1 (US 1) is a major north–south United States Numbered Highway that serves the East Coast of the United States. It runs 2,370 miles (3,810 km) from Key West, Florida, north to Fort Kent, Maine, at the Canadian border, making it the longest north–south road in the United States. US 1 is generally paralleled by Interstate 95 (I-95), though US 1 is significantly farther west and inland between Jacksonville, Florida, and Petersburg, Virginia, while I-95 is closer to the coastline. In contrast, US 1 in Maine is much closer to the coast than I-95, which runs farther inland than US 1. The route connects most of the major cities of the East Coast from the Southeastern United States to New England, including Miami, Jacksonville, Augusta, Columbia, Raleigh, Richmond, Washington, D.C., Baltimore, Philadelphia, Newark, New York City, New Haven, Providence, Boston, and Portland.

While US 1 is generally the easternmost of the main north–south U.S. Routes, parts of several others occupy corridors closer to the ocean. When the road system was laid out in the 1920s, US 1 was mostly assigned to the existing Atlantic Highway, which followed the Atlantic Seaboard Fall Line between the Piedmont and the Atlantic Plain north of Augusta, Georgia. At the time, the highways farther east were of lower quality and did not serve the major population centers. From Henderson, North Carolina, to Petersburg, Virginia, it is paralleled by I-85. Construction of the Interstate Highway System gradually changed the use and character of US 1, and I-95 became the major north–south East Coast highway by the late 1960s.

US 1 travels along the east coast of Florida, beginning at 490 Whitehead Street in Key West and passing through Miami, Hollywood, Fort Lauderdale, Boca Raton, West Palm Beach, Jupiter, Fort Pierce, Melbourne, Cocoa, Titusville, Daytona Beach, Palm Coast, St. Augustine, and Jacksonville. The southernmost piece through the chain islands of the Florida Keys, about 100 miles (160 km) long, is the two-lane Overseas Highway, originally built in the late 1930s after railroad tycoon Henry Flagler's Florida East Coast Railway's Overseas Railroad, which was built between 1905 and 1912 on stone pillars, was ruined by the 1935 Labor Day hurricane. The rest of US 1 in Florida is generally a four-lane divided highway, despite the existence of the newer I-95 not far away. Famous vacation scenic route State Road A1A is a continuous oceanfront alternate to US 1 that runs along the beaches of the Atlantic Ocean, cut only by assorted unbridged inlets and the Kennedy Space Center at Cape Canaveral. North of Jacksonville, US 1 turns northwest toward Augusta, Georgia; US 17 becomes the coastal route into Virginia, where US 13 takes over. In Florida until the 1990s, US 1 used high-contrast markers (white text on a red background).

The part of US 1 in Georgia, as it shifts from the coastal alignment in Florida to the Atlantic Seaboard Fall Line alignment in South Carolina, is generally very rural, passing through marshes and former plantations between the towns and cities of Folkston, Waycross, Alma, Baxley, Lyons, Swainsboro, and Augusta. The Georgia Department of Transportation has an ongoing plan to widen all of US 1 to four lanes with bypasses, which is more than 50 percent complete.

In South Carolina, US 1 generally serves mostly rural areas as it falls west of I-95 while the coastal areas are served by routes east of it. Starting in South Carolina, US 1 is paralleled by I-20 along the Atlantic Seaboard Fall Line through Aiken, Lexington, and Columbia to Camden and Lugoff. US 1 functions as a local two-lane road with occasional boulevard stretches. After Camden, US 1 continues northeast away from any Interstate toward Bethune, Patrick, McBee, and Cheraw with no bypasses or four-lane sections except around Cheraw through the US 52 and South Carolina Highway 9 (SC 9) concurrencies. After SC 9, it continues northward into North Carolina as a two -lane highway. The South Carolina Department of Transportation (SCDOT) has no plans to widen or bypass any US 1 alignments northeast of Camden to the North Carolina line.

Between the South Carolina line and the US 74 bypass, US 1 is a two-lane road but sees a considerable amount of truck and tourist traffic of people cutting through from the US 74/US 220 and I-73/I-74 corridor attempting to reach points south and east. US 1 goes through downtown Rockingham, with a bypass in the future plans. North of the North Carolina Highway 177 (NC 177) junction, it becomes four lanes or greater, becoming a superstreet with limited access and then becoming a limited access freeway. US 1 becomes a major artery for the state as it moves north of Rockingham. After Richmond County, it goes into Moore County with two expressway bypasses in Southern Pines, Vass, and Cameron. US 1 continues with the Jefferson Davis Highway label through Lee County and Sanford, and on to Cary and Raleigh. US 1 runs concurrently with US 64 through most of Cary, where the freeway recently underwent a major renovation and improvements that added lanes in both directions. North of Raleigh, US 1 (known as Capital Boulevard in northern Wake County) crosses I-540 and then again becomes a four-lane divided arterial to I-85 near Henderson. The North Carolina Department of Transportation (NCDOT) has begun a corridor study for this section of US 1 to determine the feasibility of transitioning the stretch of highway from I-540 to Wake Forest into a toll road. Moreover, NCDOT is planning to finish four-laning US 1 in Richmond County past NC 177 with a Rockingham bypass to the east. There are no plans from SCDOT to widen US 1 from the state line. From Henderson into Virginia, US 1 runs parallel with I-85 as a two-lane local road until the state line, where Virginia hosts a continuous third center lane for alternate passing toward US 58 before South Hill.

In the Mid-Atlantic, US 1 generally serves some of the most populated areas of the east coast. Through Virginia, US 1 is paralleled by Interstates: the remainder of I-85 to Petersburg, I-95 through Richmond and Fredericksburg to Alexandria, and I-395 into Arlington. In much of Virginia, US 1 was called the Jefferson Davis Highway by state law, although there are exceptions. South of Petersburg, it is known as Boydton Plank Road. Through some of Fairfax County and Alexandria, it is called the Richmond Highway. In February 2021, Virginia renamed all remaining portions of the Jefferson Davis Highway in the state to Emancipation Highway beginning on January 1, 2022.

US 1 crosses the Potomac River with I-395 on the 14th Street bridges and splits to follow mainly 14th Street and Rhode Island Avenue through the District of Columbia. US 1 is at the minimum of three lanes (with alternate passing) from the North Carolina state line to Petersburg with occasional four-lane divided sections. North of Petersburg is a four-lane undivided roadway at the minimum to the DC line. The route of US 1 from Petersburg to the state line is parallel with the Atlantic Seaboard Fall Line. From Petersburg onward, it is parallel with I-95. After exiting DC into Maryland, US 1 follows the Baltimore–Washington Boulevard, the first of several modern highways built along the Washington–Baltimore combined statistical area corridor; I-95 is the newest, after the Baltimore–Washington Parkway. US 1 runs through the University of Maryland, College Park, campus in College Park, Maryland. The route bypasses Downtown Baltimore on North Avenue and exits the city to the northeast on Belair Road, gradually leaving the I-95 corridor, which passes through Wilmington, Delaware, for a straighter path toward Philadelphia. Around and beyond Bel Air, US 1 is a two-lane road, crossing the Susquehanna River over the top of the Conowingo Dam before entering Pennsylvania. (Routed further north, US 1 bypasses the state of Delaware, unlike I-95.)