U.S. Route 40 (US 40) is a US highway running from Park City, Utah east to Atlantic City, New Jersey. In the US state of Delaware, it serves as a major east–west highway in northern New Castle County, Delaware, just south of Wilmington. It runs from the Maryland border east of Elkton, Maryland east to the Delaware Memorial Bridge over the Delaware River to New Jersey, at which point it is concurrent with Interstate 295 (I-295). Along the way, US 40 runs through suburban areas of Glasgow and Bear before following US 13 and I-295 around New Castle. US 40 is a multilane divided highway the entire length across Delaware.

US 40 was originally built as a state highway during the 1920s, comprising Elkton Road and a part of the Dupont Highway. When first designated, the route followed these two roads to Wilmington, where it crossed the Delaware River on a ferry to Penns Grove, New Jersey. By the 1930s, the route was rerouted at Hares Corner to head east into New Castle and cross the river on a ferry to Pennsville, New Jersey. The Delaware Memorial Bridge opened in the 1950s, and US 40 was rerouted up US 13 to the bridge approach, with Delaware Route 273 (DE 273) being extended along the former route into New Castle. By 1959, the Delaware Memorial Bridge approach became a part of I-295. Between the 1960s and 1990s, US 301 ran along the US 13/US 40 concurrency and US 301N followed US 40 between Glasgow and State Road from the 1970s to the 1980s.

US 40 enters Delaware from Maryland west of Glasgow, heading east on the four-lane divided Pulaski Highway. From the state line, the route passes through woods and areas of residential subdivisons. Upon reaching Glasgow, the road heads into commercial areas and the median of the road widens to include businesses in it. US 40 passes through more business areas and runs to the south of a field before coming to an intersection with DE 896 Bus. The median of the road narrows as it reaches an intersection with the northern terminus of US 301 and DE 896 on the eastern edge of Glasgow. Past this intersection, the route runs between Glasgow Park to the north and commercial areas to the south. US 40 passes near residential neighborhoods as it comes to an intersection with DE 72. Past this intersection, the road heads into business areas and curves to the northeast, crossing Norfolk Southern's Delmarva Secondary railroad line. The route passes through wooded areas with some development as it heads into Bear. US 40 continues through suburban areas of homes and businesses, coming to an intersection with DE 7. Past this intersection, the road continues through commercial areas and comes to an interchange with the DE 1 freeway. After this interchange, the route passes through more residential and businesses areas, passing to the south of a farm field. Farther east, US 40 heads through business areas immediately to the north of Norfolk Southern's New Castle Secondary railroad line as it reaches a directional intersection with US 13 in State Road.

At this point, US 40 heads northeast for a concurrency with US 13 on South Dupont Highway, an eight-lane divided highway. The road runs through commercial areas, coming to an intersection with DE 273 in Hares Corner. Past this intersection, US 13/US 40 passes between New Castle Airport to the northwest and businesses to the southeast as North Dupont Highway, reaching a cloverleaf interchange with the southern terminus of US 202 and DE 141 at the end of the airport property. Following this, the two routes narrow to six lanes and head through more commercial areas in Wilmington Manor, reaching an interchange with I-295.

Here, US 40 splits from US 13 by heading east along with I-295 on an eight-lane freeway maintained by the Delaware River and Bay Authority. The road has an eastbound ramp to Landers Lane before it passes between residential neighborhoods and has an interchange with DE 9. Past this interchange, the median of the freeway widens to include the Delaware River and Bay Authority headquarters, with the westbound direction coming to a toll plaza for the Delaware Memorial Bridge. I-295/US 40 continues east and passes over Norfolk Southern's New Castle Secondary before crossing the Delaware River on the twin-span Delaware Memorial Bridge, at which point both routes head into New Jersey.

US 40 in Delaware has an annual average daily traffic count ranging from a high of 92,113 vehicles at the DE 9 interchange to a low of 28,768 vehicles at the DE 896 Bus. intersection. The portion of US 40 in Delaware east of US 301/DE 896 in Glasgow is part of the National Highway System.

By 1920, what would become the Delaware portion of US 40 was a county road, with the portion along US 13 under contract to become a state highway. The Pulaski Highway and Dupont Highway portions of the road were completed as state highways by 1924. When the US Highway System was created in 1926, US 40 was designated to pass through Delaware along Elkton Road (Pulaski Highway) between the Maryland border and State Road, US 13 from State Road to Wilmington, and Christiana Avenue from US 13 to a ferry across the Delaware River which carried the route to Penns Grove, New Jersey. By 1931, US 40 was rerouted at Hares Corner to head east through New Castle and cross the Delaware River on a ferry to Pennsville, New Jersey. All of the route was a state highway by this time except the portion from Hares Corner to Basin Road. By 1936, US 40 was widened to a divided highway along Elkton Road and US 13, with the portion between Hares Corner and Basin Road being upgraded to a state highway. Construction on the Delaware Memorial Bridge and its approach to US 13 began by 1949.

By 1952, US 40 was rerouted to use the new Delaware Memorial Bridge to cross the Delaware River, being rerouted to follow US 13 north from Hares Corner to the bridge approach road. The ferry connecting New Castle to Pennsville was discontinued and DE 273 was extended along the former alignment of US 40 into New Castle. The US 40 approach to the bridge was a limited-access road with a cloverleaf interchange at DE 9 and a directional-T interchange at US 13. By 1959, the limited-access Delaware Memorial Bridge approach became a part of I-295. The interchange between I-295/US 40 and US 13/US 202 was reconstructed by 1964 to allow the westward extension of I-295 to I-95 (Delaware Turnpike). By 1966, US 301 was designated along the US 13/US 40 concurrency. US 301N was designated to follow US 40 between DE 896 and US 13 by 1971. US 301N was removed from US 40 by 1985. By 1994, US 301 was removed from the US 13/US 40 concurrency, having been rerouted to follow DE 896 to end at US 40 in Glasgow.