The South Santiam River is a tributary of the Santiam River, about 69 miles (111 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 at 44.374291°N 122.218128°W. From this source, the confluence of Sevenmile and Latiwi creeks, it flows briefly north, then generally west through the Western Cascades and Cascadia. It then flows a handful of miles before entering Foster Reservoir. At Foster Reservoir the Middle Santiam River joins the South Santiam. Downstream from the reservoir it flows west 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, about 10 miles (16 km) east of the confluence of the Santiam with the Willamette River.
U.S. Route 20 follows the valley of the river from near its headwaters in the mountains to the Willamette Valley northwest of Lebanon.
