Woodpeckers (Picidae) are broadly associated with forests but vary in sensitivity to habitat alteration, with some species restricted to mature and old-growth forests, usually because of their foraging or nesting requirements. Roosting sites have rarely been considered a critical resource for woodpecker conservation because nearly all woodpecker species roost year-round in excavated cavities similar to their nest sites. A possible exception is the globally vulnerable Helmeted Woodpecker (Celeus galeatus) of the Atlantic Forest of South America. To determine whether roosting ecology may explain their association with mature forest, we radio-tracked 12 Helmeted Woodpeckers in old-growth and selectively logged forests in Argentina, monitored roost cavities, and provided 9 roost boxes. Unlike other woodpeckers, Helmeted Woodpeckers roosted year-round in decay-formed cavities (n = 21), with perching space above the entrance, in the living trunks of large trees. Adults usually roosted alone, but after nesting (in excavated cavities), each adult roosted with one juvenile in a shared, decay-formed cavity for up to 67 days, implying that cavities need enough space for 2 birds. We did not observe use of roost boxes. Helmeted Woodpeckers travelled up to 1,482 m between nests and roosts, reused roost cavities over multiple years (sometimes changing ownership), and interacted aggressively with other cavity-nesting birds at roost cavities, indicating that suitable cavities are a scarce resource. Because logging is likely to have a strong negative impact on the availability of this resource, we urge managers in the Atlantic Forest region to retain large living trees with decay-formed cavities and to allow more selectively logged areas to become old growth.