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The existence of two seasonally distinct breeding populations of Oceanodroma storm-petrels in the Azores islands was first documented in 1996. The discovery of morphological differences between the populations led to the suggestion that they may represent cryptic sibling species. Recent mtDNA and microsatellite analysis from storm-petrel populations has considerably advanced our understanding of their taxonomic relationships. Here we present new information on the timing of breeding and moult of the two Azores populations, the extent of exchange of individuals between seasons, and diet from feather isotopes. We conclude that the hot-season Azores population should be considered a new species for which we propose the name Oceanodroma monteiroi, Monteiro's Storm-petrel. The species is both genetically distinct and genetically isolated from the sympatric cool-season population of Madeiran Storm-petrel Oceanodroma castro, and from all other populations of Oceanodroma castro in the Atlantic and Pacific Oceans examined to date. Differences in the vocalizations permit species recognition, and the extent of primary feather wear and stage of moult aids separation of the two species in the Azores, which is especially valuable during August when both attend the breeding colonies in large numbers. Feather carbon and nitrogen isotopes reveal that the diet of Monteiro's Storm-petrel differs from that of the sympatric Madeiran Storm-petrel during both breeding and non-breeding seasons, and unlike the Madeiran Storm-petrel, Monteiro's Storm-petrel appears to maintain the same foraging environment during the summer and winter months, though it shows a dietary shift to higher trophic levels during the non-breeding season. Monteiro's Storm-petrel is thought to be confined to the Azores archipelago, where it is currently known to nest on just two small neighbouring islets. The total population size was estimated at 250–300 pairs in 1999. [ABSTRACT FROM AUTHOR]