A new, critically endangered species of tarsier, one of the world's 25 most endangered primates, is described from the remote island of Siau, North Sulawesi, based on distinguishing characteristics of the tail tuft, pelage coloration, skull, and vocalizations. Siau is part of the Sangihe Island chain, a volcanic arc composed of islands that rise from the ocean floor. There is a single previous record of a tarsier from Siau; a skull in the Dresden Museum that Meyer (1897) classified with tarsiers from Sangihe Island as Tarsius sangirensis. Sangihe and Siau Islands are geologically separated by about 60 km of ocean that greatly exceeds 1,000 m in depth. Genetic data for the new species are not available, but genetic evidence indicates that its probable sister species, T. sangirensis, is a relatively ancient clade. The observations of Siau tarsier behavior seem to indicate adaptations for predator avoidance, which is intriguing, since the human inhabitants of Siau hunt and eat tarsiers for food, and tarsiers there seem to be under severe threat of extinction.