Вымерший гигантский коала: Phascolarctos stirtoni.

Extinct species of marsupial
speciesbox
| fossil range = Late Pleistocene
| image =
| image caption = restoration
| genus = Phascolarctos
| species = stirtoni
| authority = Bartholomai, 1968 and has an estimated weight of , 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 better described as a more robust koala, rather than a "giant"; in contrast, a number of Australian megafauna, such as Diprotodon and Procoptodon, were unambiguously giants, even if not compared to their closest relatives. The two koala species co existed during the Pleistocene, occupying the same arboreal niche.

The giant koala was an arboreal marsupial weighing about 13 kg, It is assumed that the giant koala was a folivore which was specialised to feed on eucalyptus leaves, like its extant relative.

Fossil remains of Phascolarctos stirtoni have been discovered in Lake Eyre and Lake Tarkarooloo basins in South Australia. It was once thought that the modern day koala was descended from the giant koala, but this has now been determined to be incorrect. Gilbert Price, of the University of Queensland, used improved dating techniques to analyse fossils of both types of koala to find that the two species were living together in the same arboreal niche. It is unclear from what the koalas descend, and why one species survived while the other became extinct. It is hypothesized that a change in climate and a restriction of food supply caused P. stirtoni's extinction. Another possible reason is hunting by humans, particularly for larger animals like P. stirtoni. The "dwarfing" hypothesis, based on the similarities of the two koalas has been used to support both ideas.