A new Malagasy species of Convolvulaceae, Bonamia ankaranensis Deroin, is described from the limestone plateaus of Ankara and Ankarana in the Majunga and Antsiranana provinces respectively. It is known from only two collections and is well characterized by its usually large thyrsoid inflorescences and its unilocular ovary at anthesis, an until now unrecognized feature in the genus, but likely evolving from an ontogenetical acceleration in the gynoecium, already demonstrated elsewhere in the family.