Hamlin County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 6,164. Its county seat is Hayti. The county was created in 1873 and organized in 1878. It was named for Hannibal Hamlin, Lincoln's first vice-president.

Hannibal Hamlin was an American attorney and politician who served as the 15th vice president of the United States from 1861 to 1865, during President Abraham Lincoln's first term. He was the first Republican vice president.