The Mangatawhiri River is a river of the Auckland and Waikato Regions of New Zealand's North Island. It flows generally southwest from its sources in the Hunua Ranges southeast of Clevedon before flowing through a system of irrigation canals at the northern edge of the Waikato Plains close to the town of Pokeno. It reaches the Waikato River close to the township of Mercer.
The upper reaches of the Mangatawhiri are dammed to form reservoirs to store water for use by Auckland City.
Under the orders of Governor George Grey, it was here that on 12 July 1863 British troops marched over and declared war on the Maori, starting the Waikato Land Wars
"Place Name Detail: Mangatawhiri River". New Zealand Geographic Placenames Database. Land Information New Zealand. Retrieved 12 July 2009.
Coordinates: 37°17′S 175°03′E / 37.283°S 175.050°E