NH 9 begins in the west where VT 9 crosses the Connecticut River from Brattleboro, Vermont, into Chesterfield, New Hampshire on the United States Navy Seabees Bridge.

One mile (1.6 km) to the east, NH 9 interchanges with US 202 and the two routes form the first of two lengthy concurrencies.

US 202 and NH 9 bypass downtown Hillsborough to the north, as well as the neighboring town of Henniker, before entering Hopkinton.

US 202 and NH 9 enter the city of Concord from the west and roughly parallel I-89 until reaching downtown.

US 202 rejoins NH 9 at this point, as does US 4, and the triplex of routes immediately crosses into Chichester, followed by Epsom.

Access to the Spaulding Turnpike exit 9 is available at this intersection NH 9 turns northeast and enters Somersworth.

NH 236 ends at the border and is not directly connected to Maine State Route 236, but the two are linked via SR 9.

The three modern state routes now form one of the longest highways in New England to retain a single number, stretching from the New York state border in Bennington, Vermont to the Canadian border in Calais, Maine, a total distance of 447.3 miles (719.9 km).

The section of NH 9 cosigned with US 202 between Hillsborough and Henniker is part of former New England Interstate Route 32.

Signed east-west, the short highway terminates at NH 9 at both ends and provides access to Spofford Lake.