U.S. Route 9 (US 9) is a north–south United States Numbered Highway in the states of Delaware, New Jersey, and New York in the Northeastern United States. It is one of only two U.S. Highways with a ferry connection ; the other is US 10. US 9 is signed east–west in Delaware and north–south on the rest of its route. The southern terminus of the route is in Laurel, Delaware, at an intersection with US 13, while the highway's northern terminus is at a junction with Interstate 87 (I-87) in Champlain, New York, where the old roadway continues north as the unsigned New York State Route 971B (NY 971B), which ends in a cul-de-sac just short of the Canadian border.

The United States Numbered Highway System is an integrated network of roads and highways numbered within a nationwide grid in the contiguous United States. As the designation and numbering of these highways were coordinated among the states, they are sometimes called Federal Highways, but the roadways were built and have always been maintained by state or local governments since their initial designation in 1926.