Horsetooth Reservoir, Recreational reservoir in Larimer County, United States

Horsetooth Reservoir is a man-made body of water in Larimer County, Colorado, stretching roughly six miles between two parallel ridges. Along the eastern shore, red-brown sandstone cliffs rise sharply, giving the waterline a distinct outline.

The Bureau of Reclamation built four earthen dams starting in 1949 as part of the Colorado-Big Thompson Project to supply water. The work created a large storage facility that channels water from the western slope of the Rockies to farmland east of the mountains.

The reservoir is named after Horsetooth Mountain, the foothills summit west of its southern end; the name refers to that mountain, not to any rock on the shore. Since the late 1960s the Dakota sandstone of the surrounding ridges has drawn bouldering climbers, and the Foothills Trail follows the eastern edge above the water.

Visitors need an entrance permit for the area and a separate permit if they plan to stay overnight at one of the campgrounds. Camping stays are limited to fourteen nights within any thirty-day window.

Cliff diving is banned across the whole reservoir, and fishing from the dams is no longer allowed for security reasons; rangers patrol for violators. Since the late 1960s the hard Dakota sandstone has drawn boulderers, who work problems on the rock rather than in the water.