In this paper, nomenclatural issues concerning nine Amaranthus taxa in the Australian flora are clarified. Lectotypes are designated for names of three currently accepted species (A. interruptus R.Br., A. rhombeus R.Br. and A. undulatus R.Br.) and two names now being considered to be taxonomic synonyms (A. lineatus R.Br. and A. macrocarpus var. pallidus Benth.). The earlier ‘holotype’ citations for the taxonomic synonym A. incurvatus Timeroy ex Gren. & Godr. and the currently accepted species A. quitensis Kunth are here considered effective lectotypifications. The holotype material for the nomenclatural synonym A. mitchellii var. grandiflorus J.M.Black is clarified. A neotype is designated for A. pallidiflorus var. viridiflorus Thell. (now considered to be a taxonomic synonym).