Marion Lake is an alpine lake located in Linn County of the U.S. state of Oregon. The lake is in central Oregon's Cascades within the Mount Jefferson Wilderness. The lake is approximately 300 acres (1.2 km2), at an elevation of 4,134 feet (1,260 m). It is the largest lake in an Oregon wilderness area and the largest lake not accessible by automobile. With the exception of large Waldo Lake, it is the largest natural lake in the Willamette National Forest on the west slope of the Central Cascade Range. It is the headwaters of Marion Creek, a tributary of the North Santiam River.