The Western Indian Ocean genus Wielandia Baill. (Phyllanthaceae, Phyllanthoideae, Wielandieae) is revised, recognizing 13 species, including the new species W. unifex Petra Hoffm. & McPherson. As a result of recent molecular phylogenetic work and the discovery of the enigmatic W. unifex, all species of Blotia Leandri and Petalodiscus Baill. are subsumed under Wielandia. The following new combinations result: W. bemarensis (Leandri) Petra Hoffm. & McPherson, W. fadenii (Radcl.-Sm.) Petra Hoffm. & McPherson, W. laureola (Baill.) Petra Hoffm. & McPherson, W. leandriana (Petra Hoffm. & McPherson) Petra Hoffm. & McPherson, W. mimosoides (Baill.) Petra Hoffm. & McPherson, W. oblongifolia (Baill.) Petra Hoffm. & McPherson, W. platyrachis (Baill.) Petra Hoffm. & McPherson, and W. tanalorum (Leandri) Petra Hoffm. & McPherson. In addition, W. bojeriana (Baill.) Petra Hoffm. & McPherson, W. danguyana (Leandri) Petra Hoffm. & McPherson, and W. ranavalonae (Leandri) Petra Hoffm. & McPherson are transferred from Savia Willd., a genus now restricted to the Neotropics. Three species, Petalodiscus pulchellus (Baill.) Pax, Savia andringitrana Leandri, and S. revoluta Scott-Elliot, and four varieties, Blotia oblongifolia (Baill.) Leandri var. louvelii Leandri, Savia andringitrana Leandri var. micrantha Leandri, S. bojeriana Baill. var. perrieri Leandri, and S. platyrachis (Baill.) Baill. var. microphylla Leandri, are subsumed under other species. Characters of flower, fruit and seed, leaf venation and anatomy, as well as pollen morphology are discussed and illustrated. A key to the 13 species of Wielandia is presented. Wielandia fadenii, previously known only from Kenya, is for the first time reported from Madagascar and the Comoro Islands. Seeds of W. danguyana, W. fadenii, and W. ranavalonae are described and illustrated for the first time, showing a wide range of embryo morphology. Distribution maps are provided for the species not previously treated as Blotia or Wielandia s. str., and IUCN Red List categories are suggested for all taxa.