A previously undescribed gecko, Nactus coindemirensis sp. nov. is reported from Gunner's Quoin, a small island off Mauritius. Nactus coindemirensis is not especially closely related to the other Mauritian member of the genus, N. serpensinsula; thus, it is likely that the precursors of these two species reached Mauritius separately, rather than both being descended from a single colonizer. Unlike other Mauritian reptiles now known only from small off-shore islands, N. coindemirensis coexists with rats, perhaps because it can evade them by taking refuge in small holes and crevices of the basalt it inhabits. This raises the possibility that it may still survive on other islands with rats, including Mauritius itself, but has escaped notice because of its size and cryptic behaviour.