The Rakaia River is one of the principal rivers of the Canterbury Plains in New Zealand's South Island. It rises in the Southern Alps, travelling 150 kilometres in a generally easterly or southeasterly direction before entering the Pacific Ocean 50 kilometres south of Christchurch.

For much of its journey, the river is a braided river, running through a wide shingle bed. Close to Mount Hutt, however, it is briefly confined to a narrow canyon known as the Rakaia Gorge.

20 kilometres from its mouth, the river is crossed by the country's longest road and rail bridges, each of which is some 1750 metres in length. The southern end of these bridges is at the small town of Rakaia.