Adult dimorphism (i.e. non-sexual dimorphism observed among sexually mature specimens) and the existence of two morphologically distinct adult stages are for the first time reported from a species of the kentrorhagid kinorhynch genus Sphenoderes. The dimorphism was found in a new species, Sphenoderes neptunus sp. nov., from the Gulf of Naples, Italy, and a formal description is provided. While dimorphism between adult life stages of kinorhynchs so far have been restricted to differences in the two posteriormost segments, the dimorphism in the new species is expressed in spine lengths throughout the entire trunk, as well as in presence or absence of cuspidate spines. The implications for comparative kinorhynch morphology and taxonomy are discussed, and it is concluded that adult dimorphism and the occurrence of more than one adult life stage always should be considered as a possibility when exploring kinorhynchs of the orders Kentrorhagata and Xenosomata. Despite the comparative challenges prompted by the adult dimorphism, species of Sphenoderes can still be distinguished by differences in shape of midventral placid and distribution of sensory spots. Conversely, spine patterns as differential characters should be used with caution, and only after ensuring that specimens belonging to the same adult stage are compared.