river in northern South Island, New Zealand. It rises in the Spenser Mountains and flows for 105 miles (169 km) between the St. Arnaud and Raglan ranges to enter Cloudy Bay of Cook Strait. Wairau Bar (Te Pokohiwi), a long spit of boulders at the river's mouth, encloses more than 15 lagoons, which had been interconnected by artificial channels before the arrival of Europeans.

Together with its principal tributaries—the Rainbow, Branch, and Waihopia rivers—the Wairau drains a basin of 1,630 square miles (4,220 square km). Its lower reaches are susceptible to flooding. The chief settlement of the valley is Blenheim, near the mouth.

In 1843 the Wairau (Maori: “Many 【or One Hundred】 Waters”) was the site of the Wairau Affray, a battle between the New Zealand Company and local Maori chiefs.