A new genus and species of fossil moonfish (Teleostei, Lampridiformes, Lamprididae) is reported on the basis of a large, partially articulated skeleton from North Otago, New Zealand. The source horizon is shelf sediments of basal Otekaike Limestone, upper Duntroonian stage, Upper Oligocene (∼26 Ma). The deep-bodied proportions of the fossil, along with features of its caudal and pectoral skeleton, support assignment to a new genus and species, described here as Megalampris keyesi, in the family Lamprididae (moonfish and opah). The fossil is notable for its large size—at an estimated 4 m total length, it is about twice the length recorded for the extant moonfish Lampris guttatus. The new form has a greatly enlarged cleithrum, which is associated with the specialized pectoral swimming characteristic of lampridids, demonstrating that this distinctive locomotory mode, along with impressively large body size, evolved in lampridids at least as early as the Oligocene. This is the first Southern Hemisphere report for a fossil lampridiform.