The giant koala (Phascolarctos stirtoni) was an arboreal marsupial which existed in Australia during the Pleistocene epoch. Fossil remains have been found in the Lake Eyre and Tarkarooloo basins of South Australia.

Phascolarctos stirtoni was about one third larger than the contemporary koala, and has had an estimated weight of 13kg, which is the same weight as a large contemporary male koala. Although considered a part of the Australian megafauna, its body mass excludes it from most formal definitions of megafauna. It is best described as a more robust koala, as the two species are practically identical.

The two koala species co-existed during the Pleistocene, occupying the same arboreal niche. The reason for the extinction of the larger of the two about 50,000 years ago is unknown.

This animal is included in lists of Australian megafauna, though it's relatively small size means it does not appear in most "formal" lists of megafauna