Molecular analysis of the newly reinstated genus Synostemon F.Muell. (Phyllanthaceae, Phyllantheae), using ITS sequence data from the now densely sampled genus, confirmed that the previously recognised ‘Queensland clade’ consists of Synostemon albiflorus (F.Muell. ex Müll.Arg.) Airy Shaw, S. sphenophyllus Airy Shaw, Sauropus podenzanae (S.Moore) Airy Shaw and the undescribed Synostemon spinescens, sp. nov. ined., the latter being validated here as S. spinosus I.Telford & J.J.Bruhl. Morphological synapomorphies for the clade are as follows: linear, apiculate anthers that are connivent but only basally connate, and thickened, clavate, recurved stigmas. A new combination is provided under Synostemon for Sauropus podenzanae as Synostemon podenzanae (S.Moore) I.Telford & Pruesapan and Sauropus convallarioides J.T.Hunter & J.J.Bruhl is formally placed in synonymy under Synostemon sphenophyllus. Descriptions, distributional data and notes on ecology and conservation status are provided for these species. Phyllanthus albiflorus F.Muell. ex Müll.Arg., the basionym of Synostemon albiflorus, is lectotypified.