South Haven is a city in Sumner County, Kansas, United States. As of the 2020 census, the population of the city was 324. South Haven was fo…South Haven is a city in Sumner County, Kansas, United States. As of the 2020 census, the population of the city was 324. South Haven was founded in 1872, and was named after South Haven, Michigan. The railroad reached South Haven in 1879.