Eureka is a city in McPherson County, South Dakota, United States. It is near the North Dakota state line. The population was 868 at the 2010 census.
Eureka was laid out in 1887, and named "Eureka", a Greek word meaning "I have found it!" Eureka is located at 45°46′9″N 99°37′19″W / 45.76917°N 99.62194°W (45.769069, -99.621953).