Sauropus elachophyllus (F.Muell. ex Benth.) Airy Shaw and S. rigidulus (F.Muell. ex Müll.Arg.) Airy Shaw are revised under the reinstated genus Synostemon F.Muell. (Phyllanthaceae) using morphological and nrITS sequence data. Sauropus decrescentifolius J.T.Hunter & J.J.Bruhl and S. elachophyllus are shown to be conspecific as Synostemon elachophyllus (F.Muell. ex Benth.) I.Telford & Pruesapan and S. elachophyllus subsp. decrescentifolius (J.T.Hunter & J.J.Bruhl) I.Telford & Pruesapan, with Synostemon elachophyllus subsp. latior (Airy Shaw) I.Telford & Pruesapan (syn. Sauropus elachophyllus var. glaber Airy Shaw p.p.) raised in rank. The ‘Top End clade’ has morphological synapomorphies of fused staminal filaments and connectives, and linear, longitudinal anthers. Four new species are named: Synostemon cowiei I.Telford & J.J.Bruhl, S. inaequisepalus I.Telford & J.J.Bruhl, S. kakadu I.Telford & J.J.Bruhl and S. nitmiluk I.Telford & J.J.Bruhl and new combinations are provided: S. crassifolius (Müll.Arg.) I.Telford & Pruesapan, S. ditassoides (Müll.Arg.) I.Telford & Pruesapan, S. dunlopii (J.T.Hunter & J.J.Bruhl) I.Telford & Pruesapan, S. filicinus (J.T.Hunter & J.J.Bruhl) I.Telford & Pruesapan, S. gracilis (J.T.Hunter & J.J.Bruhl) I.Telford & Pruesapan, S. rigidulus (F.Muell. ex Müll.Arg.) I.Telford & Pruesapan, and S. stenocladus (S.Moore) I.Telford & Pruesapan, with S. pinifolius (J.T.Hunter & J.J.Bruhl) I.Telford & Pruesapan also raised in rank.