The present study describes a new species of Cosmetocleithrum Kritsky, Thatcher and Boeger, 1986 from the gills of the catfish Trachelyopterus galeatus (Linnaeus, 1766) from the mouth of the Aguapeí River, municipality of Castilho, São Paulo State, Brazil. Cosmetocleithrum laciniatum n. sp. closely resembles Cosmetocleithrum striatuli Abdallah, Azevedo and Luque, 2012 and Cosmetocleithrum longivaginatum Suriano and Incorvaia, 1995 by the shape of dorsal and ventral bars (straight with enlarged ends), and Cosmetocleithrum gussevi Kritsky, Thatcher and Boeger, 1986 by having a Y-shaped accessory piece. The new species can be distinguished from most of its congeners by having a coiled cirrus with a 1½ counterclockwise ring with a sclerotized base possessing lateral flaps. This species is the second found to contain members of Cosmetocleithrum parasitizing the gills of Auchenipteridae. A note on the morphology of C. striatuli, a parasite of Trachelyopterus striatulus, is also presented.