A new species, Thompsonella garcia-mendozae P. Carrillo & Pérez-Calix, is described and illustrated. This taxon occurs in the Balsas River Basin of southern Michoacán, Mexico. The new species is unique in the genus Thompsonella by possessing young leaves with an aristate apex and hyaline margin, as well as sepals with a recurved apex. It is similar to T. platyphylla Rose and T. colliculosa Moran, from which it is distinguished by the possession of the lower cincinni up to 6.5 cm long and with 4 to 18 flowers. Furthermore, the flowers have pedicels up to 3 mm long. In T. platyphylla and T. colliculosa the cincinni are up to 3 and 5 cm long, respectively, and up to 12 flowers are borne, on pedicels 1 mm long or shorter. A key to the identification of species of Thompsonella is provided.