LARGE carnivorous animals, the top members of the trophic chain, are rare, and flesh-eating dinosaurs were rarer still. For years the only known giant theropods were Tyrannosaurus rex(1) and the poorly known Deinocheirus mirificus(2), both from the Northern Hemisphere, but many important new dinosaurs have been discovered in the Southern Hemisphere during the past decade, considerably increasing our knowledge of ancient ecosystems. Here me report a new giant carnivorous dinosaur from the Upper Cretaceous of northwestern Patagonia (Argentina). This new taxon, Giganotosaurus carolinii gen, et. sp, nov., is characterized by a proportionally low skull, a reduced shoulder girdle, and robust vertebrae and hind limbs. It represents a primitive evolutionary iteration of large theropods, and provides an opportunity to examine the Gondwanan dinosaur palaeocommunities and their relationships to those from Laurasia. Several characters place G. carolinii within the Tetanurae(3), and closer to Neotetanurae(4) than to Torvosauroidea(4). G. carolinii is the largest theropod ever recorded from the Southern Hemisphere, and is probably the world’s biggest predatory dinosaur, having a body 12.5 metres long and an estimated weight of 6 to 8 tonnes.