The spermatophore morphology of the endemic and monotypic hermit crab Loxopagurus loxochelis from the southwestern Atlantic is described. The spermatophores show similarities with those described for other members of the family Diogenidae (especially the genus Clibanarius), and are composed of three major regions: a sperm-filled, circular flat ampulla; a columnar stalk; and a pedestal. The morphology and size of the spermatophore of L. loxochelis, along with a distinguishable constriction or neck that penetrates almost halfway into the base of the ampulla, are characteristic of this species. The size of the spermatophore is related to hermit crab size. Direct relationships were found between the spermatophore ampulla width, total length, and peduncle length with carapace length of the hermit crab. These morphological characteristics and size of the spermatophore of L. loxochelis are species-specific, distinguishing them from other members of the family Diogenidae, and can be used to infer phylogenetic relationships among them.