Discovery Bay is an open oceanic embayment lying along the coast of eastern South Australia and western Victoria in south-eastern Australia.

Facing the Southern Ocean, Discovery Bay extends approximately 70 kilometres (43 mi) from Cape Northumberland, near Port MacDonnell, South Australia in the northwest, to Cape Bridgewater in the southeast, 20 kilometres (12 mi) west of Portland in western Victoria. Cape Northumberland is the southernmost point of mainland South Australia.^{[citation needed]} The Glenelg River is the principal inflow and enters the bay near the town of Nelson.

The bay was named by explorer Thomas Mitchell when he came down the river on 20 August 1836.

The South Australian section of the bay's coastline is protected by the Lower South East Marine Park while the Victorian section of the coast is protected in the Discovery Bay Coastal Park. The waters at the eastern end of the bay, adjacent to Cape Bridgewater, are protected by the Discovery Bay Marine National Park.