The Whangaehu River is a large river of the central North Island of New Zealand. Its headwaters are the crater lake of Mount Ruapehu on the central plateau, and its outflow is into the Tasman Sea eight kilometres southeast of Wanganui.

The river flows for 135 kilometres southward to the South Taranaki Bight near the settlement of Whangaehu.

The sudden collapse of part of the Ruapehu crater wall on December 24, 1953 led to New Zealand's worst railway accident, the Tangiwai disaster. A lahar - a sudden surge of mud-laden water - swept down the river, singificanly weakening the structure of a railway bridge at the small settlement of Tangiwai. The overnight express train between Wellington and Auckland passed over the bridge minutes later, causing it to collapse into the turbulent waters. Of the 285 people on the train, 151 were killed.