| Mangatewainui River | |
| Mouth Location: | Manawatu River |
| Subdivision Type1: | Country |
| Subdivision Name1: | New Zealand |
| Length: | 27km (17miles) |
The Mangatewainui River is a river of the Manawatū-Whanganui region of New Zealand's North Island. An upper tributary of the Manawatu River, it flows generally southeast from its sources in the Ruahine Range northwest of Norsewood, and meets the young Manawatu River 8km (05miles) east of Dannevirke.
The Mangatewainui River should not be confused with its northern neighbour, the Mangatewai River.