The Réunion Owl (Mascarenotus grucheti) was a small owl that occurred on the Mascarene island of Réunion, but became extinct before living birds could be described; it is only known from subfossil bones. It belongs to the Mascarene owls of the genus Mascarenotus, and most likely was similar to a Long-eared Owl in size and appearance, but with nearly naked legs. The Mascarene Owls were, however, more closely related to the genus Ninox. Compared to the Mauritius Owl and the Rodrigues Owl, it was the most terrestrial species of the genus, with long legs and possibly somewhat reduced flight capability; more probably though it was simply smaller than the Mauritius bird - between that species and the one from Rodrigues in size - but had equally long legs: the only suitable food available in quantity on Réunion were small birds. It can be assumed to have preyed on sleeping songbirds in the manner of the unrelated but convergent Grallistrix "stilt-owls" from Hawai'i.
As the bird is not mentioned by any contemporary report, it was either very secretive or became extinct more early than its congeners. Notably, it is not contained in the comprehensive listing of local fauna that Dubois made in 1671-72. By that time, the only introduced predators were pigs. Inferring from the ecology, it is likely that the birds succumbed to predation by rats and maybe cats as they must have been able enough for flight not to be killed off by pigs and thus only became extinct after Dubois' visit, at some date closer to the year 1700. If the bird was ground-nesting, however, it might have been extinct even by the time Dubois did not record it, but this hypothesis does not seem to agree what can be inferred from the rather long survival of its Mauritius relative.