The Sauk River is a tributary of the Skagit River, approximately 45 miles (72 km) long, in northwestern Washington in the United States. It drains an area of the high Cascade Range in the watershed of Puget Sound north of Seattle. The river is a popular destination for fly fishing. It is a National Wild and Scenic River. At Darrington the Sauk River comes so close to the head of the North Fork Stillaguamish River that boats used to portage across the divide.