Eureka is a city in Woodford County, Illinois, United States. The population was 5,295 at the 2010 census. It is the county seat of Woodford County. Eureka is part of the Peoria, Illinois Metropolitan Statistical Area. Eureka is a small community centered around the intersection of what is now U.S. 24 and Illinois 117. It is also one of the towns along the Ronald Reagan Trail, a series of roads that connect towns in northern Illinois that were of importance to Reagan's early life.