The William O. Douglas Wilderness is a designated wilderness located in central portion of the U.S. state of Washington. It includes 168,232 acres (681 km²) located between the U.S. Route 12 and State Route 410 and is jointly administered by the Snoqualmie National Forest and the Gifford Pinchot National Forest. It shares a boundary with the Mt. Rainier National Park on the west. Approximately 25 miles (40 km) of the Pacific Crest Trail travel along the Cascade Range crest between its boundaries. It contains scattered peaks, sharp ridges, steep slopes and hundreds of small lakes and potholes. Fish and wildlife are abundant here, and many minerals are found. Much of the wilderness is drained by tributaries of the Naches River.
While significant portions of the William O. Douglas Wilderness are high elevation forest, the overall topography is varied. The tallest and most visually striking peak is Mount Aix at 7,766 feet (2,367 m) with a prominence of 3,286 feet (1,002 m). The Cougar Lakes portion is characterized by high alpine lakes, and the Tumac Plateau is dotted with numerous lakes in a forest setting. The eastern edges of this wilderness drop to mid-elevation pine forest and bare ridges. The Meeks Table Natural research area, located on a basalt table mountain, is within this wilderness at its eastern boundary.