Onida is a city in Sully County, South Dakota, United States. The population was 658 as of the 2010 census. It is the county seat of Sully County.
Onida is located at 44°42′21″N 100°4′1″W / 44.70583°N 100.06694°W / 44.70583; -100.06694 (44.705939, -100.066851).
According to the United States Census Bureau, the city has a total area of 0.6 square miles (1.6 km²), all of it land.