The South Fork Wenaha River is a tributary of the Wenaha River in the U.S. state of Oregon. The river begins in the Blue Mountains south of Bone Spring in Wallowa County near its border with Union County. From there it flows generally northeast through the Wenaha–Tucannon Wilderness of the Umatilla National Forest to meet the North Fork Wenaha River. The combined forks form the main stem Wenaha, a tributary of the Grande Ronde River. Named tributaries of the South Fork Wenaha from source to mouth are Milk and Trapper creeks followed by Cougar Canyon. Then come Jaussard and Elk creeks.