Sargassum species play key ecological roles on coral reefs, yet their diversity remains poorly known. Precise identification of Sargassum species, however, is improving with molecular genetic tools, though these have yet to be applied rigorously in Singapore. Historical records list 41 species, but no more than ten were verified based on herbarium vouchers, and even fewer (five species) were confirmed in the field based on a single nuclear gene marker in a previous study. Here, we revised the diversity of Sargassum in Singapore by examining all the morphologically distinct forms collected from the local coral reef environment. A total of six morphotypes, Sargassum aquifolium (Turner) C.Argardh (1820), S. cf. granuliferum C.Argardh (1820), S. ilicifolium (Turner) C.Argardh (1820), S. swartzii C.Argardh (1820), S. polycystum C.Argardh (1824), and an undescribed taxon ‘Sargassum sp.’ (Mattio and Payri 2009), were delineated based on morphological characteristics. The morphotypes were placed in five molecular clades based on phylogenetic analyses of the nuclear ITS-2, chloroplastic partial RuBisCO operon rbcLS, and mitochondrial cox3. Sargassum cf. granuliferum, though morphologically distinct from all other species, is not phylogenetically distinct from S. polycystum. Our results provide a species list for Singapore that will be valuable for future studies on macroalgal biogeography and species-specific ecological relationships with other reef organisms, particularly corals.