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 11,233 in the 2010 census, a decline from 11,317 in the 2000 census. Spencer is famous as the home of the Clay County Fair, held annually in September, averaging over 300,000 visitors each year. The town's (now-deceased) library cat, Dewey Readmore Books, is known throughout the world.