Uniflorous species are extremely rare within the large rain forest genus Ixora. On Madagascar, however, six uniflorous species occur. Five of these belong to section Microthamnus, which is revised here. A description and illustration is provided for all species, together with a determination key. Two species, I. clandestina and I. rakotonasoloi, are newly described. The species of Microthamnus are special, not only because of their uniflorous inflorescences, but also because the ovary, calyx and base of the corolla tube of the single flower is enclosed within the stipular cone of the inflorescence-supporting leaf pair, a type of organization which is extremely rare in the Rubiaceae