Eureka is a city and county seat of Greenwood County, Kansas, United States. As of the 2010 census, the city population was 2,633, the 2015 census estimate declined to 2,450. The first settlement at Eureka was in 1857. The first post office in Eureka was established in August 1858. Eureka was laid out in 1867, and it was incorporated in 1870. The city is named from the Greek expression Eureka, meaning "I have found it".