Northfield is a town in Merrimack County, New Hampshire, United States. The population was 4,872 at the 2020 census.

Merrimack County is a county in the U.S. state of New Hampshire. As of the 2020 Census, the population was 153,808, making it the third most populous county in New Hampshire. Its county seat is Concord, the state capital. The county was organized in 1823 from parts of Hillsborough and Rockingham counties, and is named for the Merrimack River. Merrimack County comprises the Concord, NH Micropolitan Statistical Area, which in turn constitutes a portion of the Boston–Worcester–Providence, MA–RI–NH–CT Combined Statistical Area. In 2010, the center of population of New Hampshire was located in Merrimack County, in the town of Pembroke.