As part of ongoing studies on Magnolia sect. Macrophylla in North America (Mexico), unusual populations from the humid forests of Sierra Madre Oriental (central western Tamaulipas State) were found. They are here formalized as a new species, M. alejandrae. Its morphology, ecology, and conservation status are provided. A morphological comparison with similar species and diagnostic key for the species of M. sect. Macrophylla are also included. The names Magnolia ashei, M. macrophylla, and M. dealbata are lectotypified, respectively, on specimens preserved at NCU, and P and on a Zuccarini illustration. An epitype (at M) was chosen for M. dealbata.