Estimates of density and population size are fundamental in assessing population trends and ultimately in informing conservation management. Although the abundance of raptors is often expressed as indices of relative abundance, these can be poor correlates of absolute density. In 2008–2009, I calculated the absolute density and population size of Gray-backed Hawks (Pseudastur occidentalis), an endangered species and Tumbesian endemic, using line transect counts in four different habitat types in a protected area in northwestern Peru. The absolute density of Gray-backed Hawks in northwest Peru was estimated to be 0.65 individuals km−2, and the most suitable habitat for the species was located in the provinces of Manabí, Guayas, and Santa Elena in Ecuador, and Tumbes and Piura departments in Peru. The population of Gray-backed Hawks in my study areas in Tumbes was estimated to be 136, with 94% occurring in dry deciduous and deciduous forest. Because ∼60% of all detections in my study were made outside strictly protected areas, including the recently created Angostura-Faical Regional Conservation Area, conservation of the remaining, non-protected forests patches in Peru and Ecuador should be a high priority. The current global population of Gray-backed Hawks has been estimated to be between 250 and 999 birds and declining due to ongoing habitat destruction and fragmentation. Small populations in small habitat fragments, like those in my study area, have high conservation potential, provided that populations are not isolated, and hence should be the focus of constant monitoring.