The Susan River is a northeastern California river of approximately 67 miles (108 km) length that drains from an arid plateau of volcanic highlands along the Great Basin Divide to intermittent Honey Lake. The river flows from eastern Lassen County from east of Lassen Volcanic National Park generally east past Susanville and emerging into a ranching valley to enter the north end of Honey Lake. Along with Fredonyer Pass, the Susan River is the northern boundary of the Sierra Nevada.

Susanville is a city in and is the county seat of Lassen County, California, United States. Susanville is located on the Susan River in the southern part of the county, at an elevation of 4,186 feet (1,276 m). Its population is 16,728 as of the 2020 census, down from 17,947 from the 2010 census. The Susanville urban area contains 8,995 people and 4,233 households.