Dombeya formosa (Malvaceae s. l. or Dombeyaceae), a new species endemic to La Réunion, is described and illustrated. The species is distinguishable by its ovate leaves with round or slightly cordate bases, round to lightly acute apices and glabrescent abaxial surfaces, subulate and persistent stipules, umbellate inflorescences, and acute apices of floral bracts. Dombeya formosa is morphologically close to Dombeya punctata Cav. but differs from it in having ovate leaves and acute apices on floral bracts. Dombeya formosa is also morphologically similar to D. ficulnea Baill. but can be distinguished from it by its persistent and subulate stipules as well as the glabrescent adaxial surface of its leaves. Our study, focusing on 16 quantitative floral characters, demonstrates that there is a clear morphological distinction between D. formosa, D. punctata, and D. ficulnea. In comparison to the other two species, Dombeya formosa possesses smaller flowers and additionally shows cryptic dioecy along with flower-size dimorphism between sexes. Amongst the Mascarene Dombeyoideae, it is the only species distributed up to an altitude of 2,100 m. The phylogenetic position of Dombeya formosa is consistent with the morphological features and locates the species within the clade endemic to La Réunion characterized by umbellate inflorescences.