Thornton Lakes are located in North Cascades National Park, in the U. S. state of Washington. These paternoster lakes consist of three lakes located 1 to 1.75 mi (1.61 to 2.82 km) southeast of Mount Triumph. Thornton Lakes can be accessed on foot from a trailhead in Ross Lake National Recreation Area. The hike of 5.2 mi (8.4 km) includes a 2,400-foot (730 m) altitude gain and a 500-foot (150 m) descent to Lower Thornton Lake. Two other lakes lie above the first one, unofficially named Middle and Upper Thornton Lakes. A designated backcountry camping zone is located at Lower Thornton Lake.