The present paper completes a revision of the endemic Australian genus Cassinia R.Br. Cassinia subgenus Achromolaena comprises two sections, namely, section Achromolaena of seven species (C. laevis, C. arcuata, C. uncata, C. tenuifolia, C. collina, C. subtropica, and C. quinquefaria), and Cassinia section Siftonia, which contains two species (C. sifton and C. theodorii). Cassinia laevis is divided into western (C. laevis subsp. laevis) and eastern (C. laevis subsp. rosmarinifolia (A.Cunn.) Orchard, comb. et stat. nov.) taxa. Examination of the type of C. arcuata showed that this name is synonymous with C. paniculata, and applies to a relatively rare taxon with whitish capitula arranged in short erect compact panicles, and found in Western Australia, the midlands of South Australia, western Victoria and (formerly) south-western New South Wales. Furthermore, it belongs to section Achromolaena. The taxon with red to brown capitula, widespread throughout south-eastern Australia, which until now has been (incorrectly) known as C. arcuata (Sifton bush) is distinct, but lacks a published name. The name Cassinia sifton Orchard, sp. nov. is here proposed for this taxon. An unfortunate outcome of this discovery is that the sectional name Cassinia section Arcuatae, with C. arcuata as type, becomes synonymous with section Achromolaena. The new name Cassinia section Siftonia is proposed to accommodate Sifton bush (C. sifton) and its narrowly endemic sister species C. theodorii. A summary of the whole genus is provided, with keys to all taxa. Three former subspecies of C. macrocephala are raised to species rank (C. petrapendula (Orchard) Orchard, C. storyi (Orchard) Orchard, C. tenuis (Orchard) Orchard), and it is suggested that C. furtiva Orchard may be conspecific with C. straminea (Benth.) Orchard.