South Williamsport
- County:
- Lycoming County
- County Seat:
- No
- Area (mi²):
- 1.899
- State:
- Pennsylvania
South Williamsport is a borough located in Lycoming County, Pennsylvania. South Williamsport has a 2026 population of 6,176. South Williamsport is currently declining at a rate of -0.21% annually and its population has decreased by -1.2% since the most recent census, which recorded a population of 6,251 in 2020.