New Hampshire Route 25 begins at the state border, on the western bank of the Connecticut River, where it continues west as VT 25.

NH 25 crosses the southwestern corner of Benton, but lacks any road connection to the rest of the town.

To the northeast of the road sits the western end of the White Mountain chain, with Mount Moosilauke rising prominently.

The two conjoined routes parallel the Baker River, crossing it several times before entering Wentworth.

Turning gently towards the southeast, NH 25/NH 118 passes by the Plummer's Ledge Natural Area and enters the town of Rumney.

3A merges from the south and the two routes head east along Tenney Mountain Highway.

Now cosigned with US 3, NH 25 heads south along the western bank of the Pemigewasset River through Plymouth Village along Main Street, passing Plymouth State University and by a commercial district south of the university campus at the town common.

After crossing the Pemigewasset into Ashland, there is an interchange with I-93 (Exit 24) and the conjoined routes turn east into Ashland Village along Daniel Webster Highway, intersect NH 132 at Main Street and Highland Street, and turn north along the western shore of Little Squam Lake.

Entering the town of Holderness, there is an intersection with the southern end of NH 175 and the road curves to the east to follow the shore of Little Squam Lake.

After re-entering Center Harbor, NH 25 meets the eastern end of NH 25B at the central business district of Center Harbor at the northern tip of the main axis of Lake Winnipesaukee, and heads east into Moultonborough.

The two routes run concurrently eastwards along the south bank of the Bearcamp River into the village of Whittier, where NH 113 leaves to the north.

In West Ossipee, NH 25 (now known as Ossipee Mountain Highway) passes near the historic Whittier Bridge before crossing over the Bearcamp River and coming to a T-intersection with NH 16 and joining it southbound along White Mountain Highway.

While more direct than the main routing of NH 25, shortening the length between its end points by several miles, the terrain is also more rugged.