Chelsea, SD has a population of 38 and is the 28,431st largest city in the United States. The population density is 120 per sq mi which is 1009% higher than the South Dakota average and 32% higher than the national average. The median age in Chelsea is 47 which is approximately 29% higher than the South Dakota average of 37. In Chelsea, 66% of the population over 15 years of age are married, 89% speak English and 0% speak Spanish. 68% of Chelsea residents were born in South Dakota, 21% were born out of state, 0% were born outside of the United States and 11% were foreign born.