Cooperative breeding occurs in only 9% of bird species and is particularly rare among seabirds. We provide evidence that it occurs in a tropical seabird, the Red-footed Booby. Through camera monitoring of active nests, we found one nest in which a chick was raised by a trio of birds: its two parents and an immature bird. The immature bird fed the chick, guarded it, and preened it. The parents did not behave aggressively towards the immature bird when it approached the chick, suggesting that this trio was stable and composed of birds that knew each other. The growth of the chick cared for by the trio was not different from other chicks in the colony, suggesting that being fed by three birds did not result in more food provisioning. We hypothesize that the immature bird was the young of the pair that it was helping and had fledged the previous breeding season, and we discuss this in relation to the exceptionally long post-fledging parental care in this species. The prevalence of this behaviour is unknown and we recommend investigating parental care of boobies and gannets in greater detail to learn more about cooperative breeding among sulids.