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.

U.S. Route 5 (US 5) is a part of the United States Numbered Highway System that runs from New Haven, Connecticut, to the Canada–United States border at Derby Line, Vermont. In Vermont, the road runs south–north from the Massachusetts state line near Guilford to the international border. The 192.317 miles (309.504 km) that lie in Vermont are maintained by the Vermont Agency of Transportation (VTrans) and run largely parallel to Interstate 91 (I-91). US 5 also follows the path of the Connecticut River from the Massachusetts border to St. Johnsbury, where the river turns northeast while US 5 continues north. The highway serves the major towns of Brattleboro, Hartford, and St. Johnsbury, along with the city of Newport near the Canadian border.