Phylogenetic analyses of molecular sequence data obtained from the holotype of Marmosa reginaThomas, 1898, together with a reassessment of its morphological characters indicate that this species does not belong to the subgenus Micoureus as previously believed. Instead, both molecular and phenotypic data are consistent with the hypothesis that M. regina is a senior synonym of M. isthmica Goldman, 1912, in the subgenus Exulomarmosa. Because replacing isthmica with regina would create nomenclatural confusion, we recommend maintaining current usage of the former name and suppressing usage of the latter.