Lyman County is a county located in the U.S. state of South Dakota. As of the 2020 census, the population was 3,718. Its county seat is Kennebec. Lyman County was created by the Dakota Territorial Legislature on January 8, 1873, but was not organized until May 21, 1893. Its boundaries were altered in 1891, 1897, 1898, and 1916. The county was named for W. P. Lyman, a politician.