| South Branch Souhegan River | |
| Pushpin Map: | New Hampshire#Massachusetts#USA |
| Pushpin Map Size: | 250 px |
| Subdivision Type1: | Country |
| Subdivision Name1: | United States |
| Subdivision Type2: | States |
| Subdivision Name2: | Massachusetts, New Hampshire |
| Subdivision Type3: | Counties |
| Subdivision Name3: | Worcester, MA, Middlesex, MA, Hillsborough, NH |
| Subdivision Type4: | Towns |
| Subdivision Name4: | Ashburnham, MA, Ashby, MA, New Ipswich, NH |
| Length: | 5.8miles |
| Source1: | Stodge Meadow Pond |
| Source1 Location: | Ashburnham, MA |
| Source1 Coordinates: | 42.6697°N -71.8814°W |
| Source1 Elevation: | 344m (1,129feet) |
| Mouth: | Souhegan River |
| Mouth Location: | New Ipswich, NH |
| Mouth Coordinates: | 42.7311°N -71.8464°W |
| Mouth Elevation: | 927feet |
The South Branch of the Souhegan River is a 5.8adj=midNaNadj=mid[1] river located in northern Massachusetts and southern New Hampshire in the United States. It is a tributary of the Souhegan River, which flows to the Merrimack River and ultimately to the Gulf of Maine.
The South Branch begins near Mount Watatic at the outlet of Stodge Meadow Pond in the town of Ashburnham, Massachusetts, and flows north through a chain of small lakes (Marble Pond, Ward Pond, and Watatic Pond) before turning northeast into the town of Ashby, Massachusetts. North of Ashby, the river enters New Ipswich, New Hampshire, passes through a small flood control reservoir, and joins the West Branch to form the Souhegan River, just north of the intersection known as "Gibson Four Corners".