Synostemon trachyspermus (F.Muell.) I.Telford & Pruesapan (Phyllanthaceae, Phyllantheae) is shown, by morphological studies and phylogenetic analysis using nrITS DNA sequence data, to be a heterogeneous species assemblage of four species. Phyllanthus rhytidospermus F.Muell. ex Müll.Arg., with a new combination provided as Synostemon rhytidospermus (F.Muell. ex Müll.Arg.) I.Telford & Pruesapan, and Sauropus hubbardii Airy Shaw, with a new combination as Synostemon hubbardii (Airy Shaw) I.Telford & Pruesapan, are re-instated as species. Phyllanthus arnhemicus S.Moore is lectotypified and placed in synonomy under Synostemon lissocarpus (S.Moore) I.Telford & Pruesapan, which is the new combination provided for Phyllanthus lissocarpus S.Moore (syn. Sauropus lissocarpus (S.Moore) Airy Shaw). Synostemon umbrosus I.Telford & J.J.Bruhl, a rare endemic from the Kimberley, Western Australia, is named as new. The newly described S. hamersleyensis I.Telford & Naaykens, endemic to the Pilbara, Western Australia, and the north-eastern Queensland endemic Sauropus aphyllus J.T.Hunter & J.J.Bruhl are shown to be closely related; the new combination Synostemon aphyllus (J.T.Hunter & J.J.Bruhl) I.Telford & Pruesapan is provided for the latter. Sauropus sp. A of Flora of the Kimberley Region, previously included within S. trachyspermus sens.lat., shows a more distant relationship and is named as Synostemon judithae I.Telford & J.J.Bruhl. Notes on distribution, habitat, phenology, conservation status, photomicrographs of seeds and a key to identification of the species are provided.