Spencer is a city in the state of Iowa, United States, and the county seat of Clay County. It is located at the confluence of the Little Sioux and Ocheyedan rivers. The population was 11,325 in the 2020 census, an increase from 11,317 in 2000. Spencer hosts the Clay County Fair, held annually in September and averaging more than 300,000 visitors. The town's late library cat, Dewey Readmore Books, became known throughout the world before his death in 2006.