A taxonomic revision of Lysiosepalum F. Muell., a genus endemic to south-western Australia, identified five species. These comprise two currently accepted species L. involucratum (Turcz.) Druce and L. rugosum Benth., the reinstated L. hexandrum (S. Moore) S. Moore and two new species L. abollatum C.F. Wilkins (CALM Priority One) and L. aromaticum C.F. Wilkins (CALM Priority Two) that probably require recognition as Declared Rare Flora. A cladogram based on morphological data indicates that Lysiosepalum is a monophyletic genus, strongly supported by three synapomorphies unique in tribe Lasiopetaleae: calyx tube very short (lobes scarcely fused at the base); calyx lobes of alternating broad and narrow widths; epicalyx bracts equally spaced to completely enclose the mature bud. A chromosome number of n = 10 for L. involucratum is the first published record for the genus.