Ruby Mountain is a prominent mountain summit located in the North Cascades Range of Washington state. It is situated in Ross Lake National Recreation Area which is part of the North Cascades National Park Complex. The Diablo Lake Overlook along the North Cascades Highway is at the foot of the mountain. The nearest higher peak is Colonial Peak, to the southeast. Precipitation runoff on the east side of the mountain drains into Ross Lake via Ruby Creek, whereas the west side of the mountain drains into Diablo Lake via Thunder Creek.