Underhill is a town in Chittenden County, Vermont, United States. The population was 3,129 at the 2020 census.

Chittenden County is the most populous county in the U.S. state of Vermont. As of the 2020 census, its population was 168,323. The county seat is Vermont's most populous municipality, the city of Burlington. The county has over a quarter of Vermont's population and more than twice the population of Vermont's second-most populous county, Rutland. The county also has more than twice the population density of Vermont's second-most dense county, Washington. The county is named for Vermont's first governor and one of the framers of its constitution as an independent republic and later U.S. state, Thomas Chittenden.