We revise the nominal species of mouse opossums currently synonymized with Marmosa mexicana Merriam, 1897, and M. robinsoni Bangs, 1898, which include all of the trans-Andean taxa currently assigned to the nominotypical subgenus of Marmosa. In addition, we redescribe two other species that appear to be closely related to M. mexicana and M. robinsoni based on morphological or molecular citeria: M. rubra Tate, 1931, and M. xerophila Handley and Gordon, 1979. Based on first-hand examination of holotypes and other material (about 1500 specimens in total), we additionally recognize M. isthmica Goldman, 1912, and M. simonsi Thomas, 1899 (both currently synonymized with M. robinsoni), and M. zeledoni Goldman, 1917 (currently synonymized with M. mexicana), as valid species. For each of the seven species recognized as valid herein (M. mexicana, M. zeledoni, M. isthmica, M. robinsoni, M. xerophila, M. simonsi, M. rubra), we describe and illustrate diagnostic external and craniodental characters, tabulate measurement data from adult specimens, list all known examples of sympatry, and map geographic ranges based on specimens examined. The species newly recognized as valid herein, all of which occur in Central America and/or northwestern South America, substantially increase the known diversity of trans-Andean mouse opossums, but it is not currently known whether or not these represent a distinct radiation within the genus Marmosa.