Sully County
- State:
- South Dakota
- County Seat:
- Onida
Sully County's estimated 2026 population is 1,485 with a growth rate of 0.41% in the past year according to the most recent United States census data. Sully County is the 62nd largest county in South Dakota. The 2010 population was 1,377 and has seen a growth of 7.84% since that time.