Agar is a small town in Sully County, South Dakota, United States. The population was 76 as of the 2010 census. It functioned as a combined farm/ranch service center and small resort town as of 2010, its centennial year.
Agar is located at 44°50′19″N 100°4′22″W / 44.83861°N 100.07278°W / 44.83861; -100.07278 (44.838678, -100.072869).
According to the United States Census Bureau, the town has a total area of 0.2 square miles (0.4 km²), all of it land.