The Critically Endangered giant ibis Thaumatibis gigantea is the largest of 36 species in the Threskiornithidae and the national bird of Cambodia. The species historically occurred throughout Southeast Asia, but is now almost entirely restricted to northern and eastern Cambodia. The global population is estimated at 194 mature individuals. This estimate is based on incidental data and expert opinion, however, and a rigorous population census method has yet to be validated for this elusive species. We report the results of the fi rst systematic population assessment of giant ibis at a single site. Our method combines visual and auditory detections at forest pools (trapeang) and provides a cost-eff ective survey approach based on species ecology. This was tested in Prey Siem Pang Khang Lech Wildlife Sanctuary in Northeast Cambodia, previously known as Western Siem Pang Important Bird Area, one of the last strongholds of the giantibis. We estimate that 49.5 ±10 birds still occur in the site and critically review our method, suggesting refi nements. We conclude by recommending repeated surveys using a standard method at all priority protected sites for the species. This will enable the interpretation of population trends to determine the effi cacy of conservation interventions and provide an early warning, should further declines occur in this Critically Endangered species.