A rare diminutive belemnite, Pumiliobelus n. gen., is described based on the new species P. haigi and P. tumidus from the upper Gearle Siltstone (Cenomanian) of the Carnarvon Basin, Western Australia. No smaller belemnites than these species are known. That they are based on adult specimens rather than juveniles is supported by a large sample, some 80 specimens, of P. haigi which collectively show a proportional range in size typical of belemnite species in general, a group in which juveniles are conspicuously lacking in the fossil record. Rostral morphology places Pumiliobelus in the Dimitobelidae, a distinctively Austral family of Aptian—Maastrichtian age, which became progressively restricted to high latitudes through the late Cretaceous. Each species of Pumiliobelus is known from a single locality where it co-occurs with the long-ranging (Albian—Cenomanian) Dimitobelus diptychus Whitehouse of widespread distribution at mid to high latitudes in the Southern Hemisphere. Dwarfism in the Dimitobelidae, as expressed by Pumiliobelus, is considered to reflect adaptive evolution to engage opportunistic life strategies that favored small body size and rapid population turnover. The rare occurrence of dwarf Dimitobelidae indicates that such strategies were limited in both geographic range and duration. Dwarfism expressed by Pumiliobelus may relate to paedomorphosis induced by rising seawater temperatures in the mid Cretaceous.