A synopsis of the 26 Amaranthus species known to occur in Australia is presented. Amaranthus centralis J.Palmer & Mowatt and A. induratus C.A.Gardner ex J.Palmer & Mowatt are described as new; distribution maps and photographs of the type specimens are included for these new species. Amaranthus undulatus R.Br. is the earliest correct name for the taxon currently known as A. pallidiflorus F.Muell., and lectotypes are selected for A. clementii Domin, A. leptostachyus Benth., A. macrocarpus Benth. and A. mitchellii Benth. A key to all species in Australia is presented.