I describe a new species of Micrastur (forest-falcon) from the rainforests of Brazil and adjacent northeastern Bolivia. Initial detection of the cryptic new taxon was enabled through hearing its distinctive voice, notably different from any of its congeners. Several specimens of the undescribed species subsequently were located in several museums; because the new species closely resembles M. gilvicollis (Lined Forest-Falcon), these specimens had remained unrecognized for more than a century. The new taxon not only has a vocal repertoire that differs from those of congeners, M. gilvicollis, M. plumbeous (Plumbeous Forest-Falcon), and M. ruficollis (Barred Forest-Falcon), but it also exhibits subtle yet consistent morphological distinctions that distinguish it from all other forest-falcons. This elusive raptor inhabits humid terra firme forest in southeastern Amazonia, and a disjunct population exists in the Atlantic rainforests of eastern Brazil (the latter known only from historic specimens) and merits great conservation concern. I summarize current information on the new species’ distribution, offer insight into its natural history and conservation, and discuss its systematic relationships within the genus Micrastur.