Based on extensive studies of fresh material of known origin, all descriptions and type-specimens of all species of the genus Trichodiadema Schwantes (Mesembryanthemoideae, Aizoaceae) for the new Lexicon of Aizoaceae (Hartmann edition, in prep.) the delimitation of the genus is revised. The term “diadem” is defined. Useful characteristics of leaf anatomy and morphology, as well as significant flower and fruit characters serve as criteria, presenting a basis for a complete revision of the genus.