The South Fork Eel River is the largest tributary of the Eel River in north-central California in the United States. The river flows north from Laytonville to Dyerville/Founders' Grove where it joins the Eel River. The South Fork drains a long and narrow portion of the Coast Range of California in parts of Mendocino and Humboldt counties. U.S. Route 101 follows the river for much of its length.