The Mangatewainui River is a river of the Manawatà «-Whanganui region of New Zealand's North Island. An upper tributary of the Manawatà « River, it flows generally southeast from its sources in the Ruahine Range northwest of Norsewood, and meets the young Manawatà « River east of Dannevirke.
The Mangatewainui River should not be confused with its northern neighbour, the Mangatewai River.