The Waitaki River is a large braided river on the South Island of New Zealand. It drains the Mackenzie Basin and runs 209 kilometres (130 mi) south-east to enter the Pacific Ocean between Timaru and Oamaru on the east coast. It starts at the confluence of the Ōhau and Tekapo rivers, now at the head of the artificial Lake Benmore, these rivers being fed by three large glacial lakes, Pukaki, Tekapo, and Ōhau at the base of the Southern Alps. The Waitaki flows through Lake Benmore, Lake Aviemore and Lake Waitaki, these lakes being contained by hydroelectric dams, Benmore Dam, Aviemore Dam and Waitaki Dam. The Waitaki has several tributaries, notably the Ahuriri River and the Hakataramea River. It passes Kurow and Glenavy before entering the Pacific Ocean. The River lends its name the Waitaki District on the south side of the river bank.

A braided river consists of a network of river channels separated by small, often temporary, islands called braid bars or, in British English usage, aits or eyots.