Waterloo is a city in Linn County, Oregon, United States. The population was 229 at the 2010 census. The city's name, a joking reference to the Battle of Waterloo, stemmed from a 19th-century court decision that settled a land dispute at Kees Mill along the South Santiam River. After the dispute ended, Kees Mill became known as Waterloo. The federal government established a post office at Waterloo on January 5, 1875. S. D. Granger was its first postmaster. The post office closed in 1974.