As the global extinction crisis escalates, an increasing number of species are declining to a state of ‘extreme rarity’, persisting as small, geographically restricted populations reduced to a handful of individuals. Conservation decision-making for such species must be rapid, as delays can mean the difference between extinction and recovery, and decisions must be grounded in evidence to succeed. Paradoxically, for species on the edge of extinction, robust data are often unavailable and their very rarity makes data collection challenging. The Critically Endangered Hainan gibbon (Nomascus hainanus) is the world’s rarest ape, constituting one population of approximately 25 individuals restricted to one protected area, Bawangling National Nature Reserve on Hainan Island, China. Using the case of the Hainan gibbon, I demonstrate that despite the species’ tiny population size and highly restricted distribution, by adopting a multifaceted approach to clarify key features of the ecology, behaviour, and genetic condition of the last surviving population, it is possible to develop a robust evidence base that can be used to inform conservation planning. I reveal that the Hainan gibbon has a smaller home range than previously thought, the species’ genetic diversity has declined by ~30% from historical levels, and individuals in the remaining population are closely related. Predictive models within a phylogenetic framework suggest that large, polygynous groups may be evolutionarily characteristic for the species. Together, these findings indicate that intensive, carefully planned management is essential for this species, and that an evidence-based approach to the conservation of even extremely rare species is feasible.