Allyn River, a perennial stream[2] of the Hunter River catchment, is located in the Hunter region of New South Wales, Australia.

Allyn River rises on Allyn Range, on the slopes of the Gondwana Rainforests Barrington Tops, west of Careys Peak, and flows generally southeast, joined by seven minor tributaries, before reaching its confluence with the Paterson River near Vacy; descending 640 metres (2,100 ft) over its 82-kilometre (51 mi) course.

[1] The course of the river flows through World Heritage listed high elevation rainforest, noted for its Antarctic Beech; and then through lower elevation subtropical rainforest, including trees such as Red Cedar and Small leaf fig.

[3] Some of the River Oak growing beside the stream are over 50 metres (160 ft) in height.

[4] The Allyn River valley is the traditional territory of the Gringai clan of the Wonnarua people,[5][6] a group of Indigenous Australians.