Van Diemen Gulf () is a gulf between Arnhem Land, the Cobourg Peninsula and Melville Island in northern Australia. It joins the Timor Sea in the west by the Clarence Strait near the city of Darwin. It joins the Arafura Sea in the north by Dundas Strait between Melville Island and Cobourg Peninsula. It covers an area of about 14,000 km².
Rivers flowing into the Gulf include the South Alligator River, the East Alligator River and the Adelaide River.
Abel Tasman mapped the northern entrance to the Gulf, Dundas Strait, in 1644. The Gulf is named after the Dutch Governor of Batavia, Anthony Van Diemen.
The waters in the Gulf near the Cobourg Peninsula are protected by the Garig Gunak Barlu National Park.