Six extant species of non-human great apes are currently recognized: Sumatran and Bornean orangutans, eastern and western gorillas, and chimpanzees and bonobos 1. However, large gaps remain in our knowledge of fine-scale variation in hominoid morphology, behavior, and genetics, and aspects of great ape taxonomy remain in flux. This is particularly true for orangutans (genus: Pongo), the only Asian great apes and phylogenetically our most distant relatives among extant hominids 1. Designation of Bornean and Sumatran orangutans, P. pygmaeus (Linnaeus 1760) and P. abelii (Lesson 1827), as distinct species occurred in 2001 1, 2. Here, we show that an isolated population from Batang Toru, at the southernmost range limit of extant Sumatran orangutans south of Lake Toba, is distinct from other northern Sumatran and Bornean populations. By comparing cranio-mandibular and dental characters of an orangutan killed in a human-animal conflict to those of 33 adult male orangutans of a similar developmental stage, we found consistent differences between the Batang Toru individual and other extant Ponginae. Our analyses of 37 orangutan genomes provided a second line of evidence. Model-based approaches revealed that the deepest split in the evolutionary history of extant orangutans occurred ∼3.38 mya between the Batang Toru population and those to the north of Lake Toba, whereas both currently recognized species separated much later, about 674 kya. Our combined analyses support a new classification of orangutans into three extant species. The new species, Pongo tapanuliensis, encompasses the Batang Toru population, of which fewer than 800 individuals survive. Display omitted •We describe a new species of great apes, the Tapanuli orangutan Pongo tapanuliensis•Genomic analyses corroborate morphological distinctiveness of P. tapanuliensis•P. tapanuliensis comprises the oldest evolutionary lineage in the genus Pongo•With fewer than 800 individuals, P. tapanuliensis is among the most endangered great apes Nater et al. describe a new great ape species, the Tapanuli orangutan Pongo tapanuliensis. An isolated population from Batang Toru is highly distinct from the northern Sumatran and Bornean species, based on morphological variation, corroborated by population genomic analyses. Fewer than 800 individuals of P. tapanuliensis survive in the wild.