The South Fork Umatilla River is a tributary of the Umatilla River in Umatilla County in the U.S. state of Oregon. Its headwaters lie in the Umatilla National Forest in the Blue Mountains of northeastern Oregon near Pileup Saddle and Black Mountain. The South Fork flows generally north between Goodman Ridge and Bobsled Ridge to meet the North Fork Umatilla River near Graves Butte. Together the forks form the main stem Umatilla.