Barrett Mountain stands in south-central New Hampshire within New Ipswich, on the Wapack Range ridgeline between Kidder Mountain and New Ipswich Mountain. The 22-mile Wapack Trail crosses it, and because its summit is mostly wooded, it is known more as part of the ridge and trail corridor than as an open peak. A cross-country ski area occupies its north side. The mountain is also notable for dividing several drainage systems: water from its east side flows toward the Souhegan and Merrimack to the Atlantic, from the northwest side to the Gridley and Contoocook before reaching the Merrimack, and from the southwest side to the Millers and then the Connecticut River and Long Island Sound.