The giant koala (Phascolarctos stirtoni) was an arboreal marsupial which existed in Australia during the Pleistocene epoch. Phascolarctos stirtoni was about one third larger than the contemporary koala,[1] and has had an estimated weight of 29 lb (13 kg), which is the same weight as a large contemporary male koala.[2] 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, rather than a "giant"; whereas a number of Australian megafauna, such as Diprotodon and Procoptodon goliah, were unambiguously giants.

The two koala species co-existed during the Pleistocene, occupying the same arboreal niche.[1] The reason for the extinction of the larger of the two about 50,000 years ago is unknown.