The Great Australian Bight is a large oceanic bight, or open bay, off the central and western portions of the southern coastline of mainland Australia.

There are two definitions for its extent—one by the International Hydrographic Organization (IHO) and another by the Australian Hydrographic Service (AHS). The bight is generally considered part of the Indian Ocean, although the AHS classifies it as part of the Southern Ocean. Its coastline is characterized by cliff faces and rocky capes, making it an ideal location for whale-watching.

This oceanic feature was first encountered by European explorers in 1627, and was accurately charted by English navigator Matthew Flinders in 1802. The Great Australian Bight came into existence about 50 million years ago when the supercontinent Gondwana broke apart, separating Antarctica from Australia.

The Bight's waters are highly biodiverse, especially in zooplankton, due to specific ocean currents. However, more research is needed to fully understand the region's complex ecosystems and the potential impacts of human activities, such as resource extraction. Southern bluefin tuna are a popular fishing target in the Bight, and there have been attempts to explore the area for oil and gas since the 1960s. However, these proposals have faced significant opposition due to the potential environmental impacts.

Two definitions of the extent are in use – one used by the International Hydrographic Organization (IHO) and the other used by the Australian Hydrographic Service (AHS).

The IHO defines the Great Australian Bight as having the following limits:

On the North. The south coast of the Australian mainland.

On the South. A line joining West Cape Howe (35°08′S 117°37′E / 35.133°S 117.617°E / -35.133; 117.617) Australia to South West Cape, Tasmania.