The Ruamahanga River runs through the southeastern North Island of New Zealand.

From there it runs firstly south and then southwest for 130 kilometres (81 mi) before emptying into the Cook Strait.

The towns of Masterton and Martinborough are close to the banks of the river.

[2] In its lower reaches, the river meanders across a large floodplain, culminating in the wetlands around the edges of Lake Wairarapa.

The river drains at Palliser Bay 10 kilometres (6 mi) further south.