Here we describe Australia's first known Mesozoic mammal and the first known early Cretaceous mammal from Gondwanaland. Steropodon galmani n. gen. and sp., discovered in early Cretaceous sediments at Lightning Ridge, New South Wales, Australia, appears to represent an ornithorhynchid-like monotreme. This discovery represents the first record of a fossil mammal from Australia that is older than 22.4+/-0.05 Myr1,2 and the specimen is, by more than 85Myr, the oldest known monotreme. As the oldest monotreme, it will necessitate a radical revision of present understanding about dental homology in the middle Miocene Obdurodon insignis, the only fossil monotreme previously known to have had teeth3. The structure of S. galmani supports one current view4 that monotremes, one of three groups of living mammals (the other two being marsupials and placentals), are phylogenetically close to the other groups of living mammals.