Cryptic benthos such as the clionaid sponge genus Cliothosa can be inadequately sampled and described. Historic descriptions and diagnoses are largely insufficient and have resulted in a confused taxonomy. We assessed accepted and invalid Cliothosa and some Cliona species. A new genus diagnosis now also considers papillar structures and erosion traces. Extended inhalant papillae have a villar surface. We propose that the villar surface of inhalants and the typically large erosion chambers are deciding characters in the field. In the laboratory, size-variable, often subterminal tylostyles, and microscleres are diagnostic, but the latter can be rare or missing. If present, they belong to three groups: type I—‘conventional’ helical spirasters with slightly recurving spines; type II—ramose, sharp-spined amphiasterose or spirasterose microscleres and/or their reduced versions; and type III—rare minute rugose spirasters. Our revision resulted in 18 distinct Cliothosa species, tripling the existing record: Cliothosa adriatica comb. nov. (with synonym Papillella quadrata sensu von Lendenfeld), Cliothosa argus comb. nov., Cliothosa aurivillii stet. (with synonym Ridleia peleia), Cliothosa chilensis comb. nov., Cliothosa delitrix comb. nov. taxon inq. (with synonym Cliona laticavicola), Cliothosa dichotoma incert. sedis, Cliothosa hancocki stet., Cliothosa investigatoris taxon inq., Cliothosa laevicollis comb. nov., Cliothosa liangae comb. nov., Cliothosa nodulosa comb. nov., Cliothosa quadrata stet., Cliothosa ramosa sp. reconcil. comb. nov., Cliothosa rhodensis comb. nov., Cliothosa seurati sp. reconcil. sp. typorum, Cliothosa tylostrongylata stet., Cliothosa utricularis comb. nov., and Cliothosa warreni sp. reconcil. comb. nov. taxon inq. Cliona cretensis sp. reconcil. incert. sedis and Cliona thoosina incert. sedis may be part of Cliothosa as well, but more information is needed. We provide a key for all the species we have considered.