The Arnold River is a river in the north of the Northern Territory of Australia.

The Arnold River rises about 30 kilometres (19 mi) northeast of the Bullwaddy Conservation Reserve and north of the Carpentaria Highway. It flows to the town Minamia (Cox River) in the Aboriginal reserve Alawa and then turns northwest and flows about 15 kilometres (9.3 mi) south of Hodgson Downs in the Hodgson River. The river is joined by two tributaries including the Williams and Horse creeks and flows through a series of lagoons and waterholes before reaching its river mouth.

The river is named for Richard Aldous Arnold, a pastoralist and politician, who purchased a pastoral lease in the area in 1895. The lease was a part of the Hodgson Downs cattle station. He sold his interest in 1905 and continued his political career as Clerk of the Legislative Assembly of New South Wales.

• List of rivers of Northern Territory