| South Branch Ashuelot River | |
| Pushpin Map: | New Hampshire#USA |
| Pushpin Map Size: | 250 px |
| Subdivision Type1: | Country |
| Subdivision Name1: | United States |
| Subdivision Type2: | State |
| Subdivision Name2: | New Hampshire |
| Subdivision Type3: | County |
| Subdivision Name3: | Cheshire |
| Subdivision Type4: | Towns |
| Subdivision Name4: | Troy, Marlborough, Swanzey |
| Length: | 17.4miles |
| Source1: | Confluence of Rockwood Brook and Quarry Brook |
| Source1 Location: | Troy |
| Source1 Coordinates: | 42.8275°N -72.1783°W |
| Source1 Elevation: | 291m (955feet) |
| Mouth: | Ashuelot River |
| Mouth Location: | Swanzey |
| Mouth Coordinates: | 42.8892°N -72.2825°W |
| Mouth Elevation: | 141m (463feet) |
| Tributaries Left: | Bridge Brook, Martin Brook |
| Tributaries Right: | Shaker Brook, Forbush Brook |
The South Branch of the Ashuelot River is a 17.4adj=midNaNadj=mid[1] river located in southwestern New Hampshire in the United States. It is a tributary of the Ashuelot River, itself a tributary of the Connecticut River, which flows to Long Island Sound.
The South Branch begins in the town of Troy, New Hampshire, at the confluence of Rockwood Brook and Quarry Brook. The river flows in a zigzag manner northwest, then southwest, then north to the Ashuelot River in Swanzey. On the river's northwest-flowing leg, it drops rapidly as it cuts a deep, narrow valley through the highlands of Troy and Marlborough. New Hampshire Route 12 and an abandoned railroad line follow the river along this section. When the river turns southwest, the valley widens, except for a brief section where the river passes the village of East Swanzey. The final, north-flowing leg of the river is through a broad, flat intervale, where the river makes numerous tight meanders.