The Allen River is a river in the West Coast district of the South Island of New Zealand. It drains the south side of the Allen Range, elevations to 1,450 metres (4,760 ft), which forms part of the boundary between West Coast and Tasman districts. The river has left and right branches for half its length, and the right branch has a major tributary stream in a valley between the two branches. The united river flows south through a narrow gorge and is joined by the 200 metres (660 ft) upstream of its confluence with the North branch of the Mōkihinui River.