The Blue Mountains are a mountain range located primarily in western Oregon state and southwestern Washington state. This range covers a large area of Oregon, some 15,000 square miles from Pendleton, Oregon in the north, to the Snake river along the western state line of Oregon which borders with Idaho, west to the Lava Plains, and south to the Northern Basin Ecoregion. In Washington, the area is much smaller and located on the southern state line in the extreme southwest corner.

The Blue Mountains are an uplift mountain range and contain some of the oldest rocks in Oregon, approximately 400 million years old. Geologically, the Blue Mountains are part of the Columbia River Plateau in eastern Oregon and Washington and west as far as the Cascades Foothills Ecoregion.

The Blue Mountains are composed of several other mountain ranges including: Ochoco Mountains, Maury Mountains, Greenhorn Mountains, Aldrich Mountains, Strawberry Range, Elkhorn Mountains, and the Wallowa Mountains.