Abstract: Tyrannosaurs, giant predatory dinosaurs from the end of the Cretaceous, are among the most intensively researched and best-known groups of dinosaurs. Despite this, their relationships and systematics are highly controversial, and the number of tyrannosaur species occurring in the latest Cretaceous in North America is debated. An ongoing debate concerns the status of Nanotyrannus lancensis, which has variously been interpreted as a distinct taxon of small-bodied tyrannosaur or a juvenile of the coeval Tyrannosaurus rex. Here, we review multiple lines of evidence and show that the totality of evidence strongly supports recognition of Nanotyrannus as a distinct species: 1. The high diversity of Late Cretaceous tyrannosaurs and predatory dinosaurs in general is consistent with the idea that more than one species lived in the late Maastrichtian of Western North America; 2. Nanotyrannus shows few if any diagnostic characters allowing referral specifically to Tyrannosaurus or even Tyrannosaurinae, but is differentiated from T. rex by at least 77 morphological characters, while intermediate forms, combining characteristics of Nanotyrannus and T. rex, remain unknown; 3. Histological evidence shows that individuals previously referred to Nanotyrannus lancensis show (i) skeletal fusions consistent with maturity, (ii) skull bone textures consistent with maturity, (iii) slow growth rates relative to T. rex, (iv) decelerating growth in their final years of life, and (v) growth curves predicting adult body sizes of ~1500 kg or less, implying that these animals are young adults, not juveniles of Tyrannosaurus; 4. Juveniles of other tyrannosaurids, including Gorgosaurus and Tarbosaurus, do not show the kinds of changes proposed for a Nanotyrannus-Tyrannosaurus growth series, suggesting the Nanotyrannus morphology cannot simply be explained as the result of immaturity; 5. Small T. rex exist, comparable in size to Nanotyrannus, which exhibit diagnostic features of Tyrannosaurus, and which differ from Nanotyrannus; 6. Phylogenetic analysis suggests that Nanotyrannus is not a tyrannosaurid. Taken together, the totality of the evidence rejects referral of Nanotyrannus to T. rex. Phylogenetic analysis tentatively supports placement of Nanotyrannus outside of Tyrannosauridae as a non-tyrannosaurid member of Tyrannosauroidea. Tyrannosaur diversity appears to have been higher than previously appreciated in the latest Cretaceous before the K-Pg extinction. The difficulties in recognizing species based on fossils alone mean that paleontologists may be systematically biased towards underestimating the species diversity of ancient ecosystems.