The Aucilla River rises close to Thomasville, Georgia, USA, and passes through the Big Bend region of Florida, emptying into the Gulf of Mexico at Apalachee Bay. The river is long and has a drainage basin of . The Wacissa River is a tributary. In Florida, the Aucilla River forms the eastern border of Jefferson County, separating it from Madison County on the northern part, and from Taylor County to the south. The lower part of the river disappears underground and reappears several times, and is known as the Aucilla River Sinks. The Aucilla River is a rich source of late Pleistocene and early Holocene animal bones and human artifacts, and is the subject of the Aucilla River Prehistory Project, which includes the Page-Ladson prehistory site. During the first Spanish period in Florida the Aucilla River was the boundary between the Apalachee people and the Timucua-speaking Yustaga (or Uzachile) people.