The Waimakariri River is a river in Canterbury, in the South Island of New Zealand. It flows for 150 kilometres in a generally southeastward direction from the Southern Alps to the Pacific Ocean.

The river rises on the eastern flanks of the Southern Alps, eight kilometres southwest of Arthur's Pass. For much of its upper reaches, the river is braided, with wide shingle beds. As the river approaches the Canterbury Plains, it passes through a belt of mountains, and is forced into a narrow canyon (the Waimakariri Gorge), before reverting to its braided form for its passage across the plains. It finally enters the Pacific north of Christchurch, near the town of Kaiapoi.

Geological evidence indicates that the river mouth has been very mobile, at times flowing through the current location of Christchurch and even flowing into Lake Ellesmere south of Banks Peninsula for a time.

The river is known colloquially in Canterbury as "The Waimak".