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, and has had an estimated weight of 13 kg (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, 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. The reason for the extinction of the larger of the two about 50,000 years ago is unknown.