The South Santiam River is a tributary of the Santiam River, approximately 60 mi (97 km) long, in western Oregon in the United States. It drains an area of the Cascade Range into the Willamette Valley east of Corvallis.

It rises in the Cascades in southeastern Linn County in the Willamette National Forest. It flows briefly north, then generally west through the mountains, past Cascadia to the Foster Reservoir , where it receives the Middle Santiam River from the northeast as an arm of the reservoir. Downstream from the reservoir it flows east past Sweet Home, where it turns northwest, passing through the foothills into the Willamette Valley near Lebanon. It joins the North Santiam River from the southeast to form the Santiam approximately 15 mi (24 km) northeast of Lebanon. The confluence is approximately 10 mi (16 km) east of the confluence of the Santiam with the Willamette River.

U.S. Highway 20 follows the valley of the river from near its headwaters in the mountains to the Willamette Valley northwest of Lebanon.