New Hampshire Route 123 is a 63.055-mile-long (101.477 km) secondary north–south state highway in southwestern New Hampshire. The southern terminus of the route is at the Massachusetts state line in Mason where, as Mason Road, the road continues as an unnumbered local road in the town of Townsend. The northern terminus, as signed, is at the Connecticut River, where the highway continues west for 0.313 miles (0.504 km) to U.S. Route 5 in Westminster, Vermont, as Vermont Route 123. Route logs, however, place the terminus at New Hampshire Route 12 in Walpole.

Mason is a town in Hillsborough County, New Hampshire, United States. The population was 1,448 at the 2020 census. Mason, together with Wilton, is home to Russell-Abbott State Forest.