The Lost River is a 4.0-mile-long (6.4 km) stream in the White Mountains of New Hampshire in the United States. It is a tributary of Moosilauke Brook, part of the Pemigewasset River watershed leading to the Merrimack River. The river flows southeast from Kinsman Notch to Jackman Brook, where the two streams form Moosilauke Brook, which continues northeast through the granite gorge of Agassiz Basin and joins the Pemigewasset River in the village of North Woodstock. New Hampshire Route 112 follows the Lost River from Kinsman Notch to Moosilauke Brook.