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. The city was founded in the year 1855. Eureka is part of the Peoria, Illinois Metropolitan Statistical Area. According to the United States Census Bureau, the city has a total area of 2.7 square miles (7.0km2), of which, 2.7 square miles (7.0km2) of it is land and 0.1 square miles (0.26km2) of it (1.82%) is water.