The Withlacoochee River (South) originates in central Florida's Green Swamp, east of Polk City. It flows west, then north, and then turns northwest and finally west again before it empties into the Gulf of Mexico near Yankeetown. The river is 86 miles (138 kilometers) long and has a drainage basin of 1,170 square miles (3,030 square kilometers). It is believed to have been named after the river to the north.
The Withlacoochee River forms most of the boundary between Polk County and Sumter County, then flows through Pasco County and Hernando County, and then forms part of the boundary between Hernando County and Sumter County, and all of the boundary between Citrus County and Sumter County, between Citrus County and Marion County and between Citrus County and Levy County (including Lake Rousseau). The largest city close to the river is Dade City.