Crystal Lake is a 455-acre (1.8 km²) water body located in Belknap County in the Lakes Region of central New Hampshire, United States, in the town of Gilmanton. Crystal Lake is at the head of the Suncook River watershed. Water flows into Crystal Lake from the Belknap Range through Manning and Sunset lakes to the north. Water from Manning Lake enters Crystal Lake via Nelson Brook. Other waterways flowing into Crystal Lake include Wasson Brook and Mill Brook. Water flows out of a dam from the southern end of Crystal Lake via the Suncook Lakes and Suncook River to the Merrimack River. Shoreline development along Crystal Lake consists primarily of summer cottages, with a few year-round residents.