The aridification of the Australian continent led to the diversification of mygalomorph spiders in the Miocene, but a depauperate fossil record has made it difficult to investigate evolution across this epoch. Here, we describe the first fossil barychelid spider (Araneae: Barychelidae) in the world and the second fossil mygalomorph spider from Australia. It is placed as a new genus and species (Megamonodontium mccluskyi gen. et sp. nov.). Megamonodontium resembles Monodontium Kulczyński, 1908, a genus that persists in rainforests through Singapore, Indonesia and Papua New Guinea. The new specimen is the second largest spider fossil in the world and is approximately five times larger than extant Monodontium. The fossil shows that this lineage once occupied mesic rainforest habitats in Australia but has since been replaced by other spiders.