According to the U.S. Census Bureau, the county has a total area of , of which is land and (7.8%) is water [REF].
The northeastern third of Pierce County, bordered by a line from just west of Mershon to just south of Bristol, then south to just north of Blackshear, and then heading due east, is located in the Little Satilla River sub-basin of the St. Marys River-Satilla River basin.