Seneca, SD has a population of 9 and is the 28,989th largest city in the United States. The population density is 21 per sq mi which is 99% higher than the South Dakota average and 76% lower than the national average. The median age in Seneca is 74 which is approximately 102% higher than the South Dakota average of 37. In Seneca, 78% of the population over 15 years of age are married, 100% speak English and 0% speak Spanish. 100% of Seneca residents were born in South Dakota, 0% were born out of state, 0% were born outside of the United States and 0% were foreign born.