Lithothamnion (Corallinaceae, Rhodophyta) is represented in southern Australia by at least two species: L. muelleri, the type species of the genus, and L. indicum. Detailed accounts are provided along with a key to the species, and information on distribution, seasonality and habitat, nomenclature and synonymy, and infraspecific taxonomy. Two characters associated with tetrasporangial conceptacle roof anatomy are considered diagnostic of the two southern Australian species. Twenty-five characters used by other authors to separate species of Lithothamnion were examined; none were found to be diagnostic of L. muelleri or L. indicum, and only one (plant habit) was sometimes found to be useful as an ancillary character. None of the measured characters assessed could be used in a diagnostic or ancillary manner. The status and disposition of 22 other specific and infraspecific taxa ascribed to Lithothamnion and reported from southern Australia are summarised; 3 are considered synonyms of L. indicum or L. muelleri, 12 are referable to other genera, 6 are considered to be of uncertain status, and 1 constitutes an unconfirmed record.