Colonial Peak is a mountain in the North Cascades of Washington, United States. It rises steeply from the southwest bank of Thunder Arm of Diablo Lake, one of the artificial reservoirs on the Skagit River. It is part of a group of peaks that form the northern end of a chain running south through prized climbing destinations such as Eldorado Peak, Forbidden Peak, and Goode Mountain. Colonial Peak takes its name from the Colonial mining claim on its slopes.