U.S. Route 40 (US 40) is a US highway running from Silver Summit, 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, just south of Wilmington. It runs from the Maryland state line west of Glasgow 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 passes through suburban areas in Glasgow and Bear before running concurrent with US 13 and I-295 around New Castle. US 40 is a multilane divided highway the entire length across Delaware, with the section concurrent with I-295 a freeway.

New Castle County is the northernmost of the three counties of the U.S. state of Delaware. As of the 2020 census, the population was 570,719, making it the most populous county in Delaware, with nearly 60% of the state's population of 989,948. The county seat is Wilmington, which is also the state's most populous city.