Valonia cladophora Kutzing was described from New Caledonia in the 1860s, on the basis of a single specimen, but has not since been observed or studied. Preliminary observations of the type specimen led to its transfer to the genus Cladophora and because of ICBN regulations the species required a new name, C. mamillata Leliaert in Millar & Prud'homme van Reine. In this paper, a detailed description of this poorly known taxon is provided. Cladophora mamillata is characterized by mamillate elevations on the cross-walls, a unique feature in the genus. Morphologically, C mamillata falls into the Cladophora section Aegagropila, where it closely resembles C. catenata (Linnaeus) Kutzing, a widespread species in the Indo-Pacific and Atlantic Oceans.