A new genus and species of Cylindropsyllidae (Copepoda, Harpacticoida), collected from shallow subtidal sand in the Southern Bight of the North Sea, is described and illustrated. Willemsia calceola gen. et sp. n. exhibits a unique kind of sexual dimorphism on the male P3 not observed thus far in any other harpacticoid. It is further distinguished from all known Cylindropsyllinae by the absence of an isolated midventral copulatory pore, the facies of the P2-P3 in the female and the peculiar shape and orientation of the fifth pair of legs in both sexes. Based on the sexual dimorphism of the P2 exopod and P3 endopod, it is suggested that W. calceola shares a sistergroup relationship with Stenocaris kerguelenensis BODIOU, 1977 which is transferred to a new genus Navalonia. Boreopontia heipi WILLEMS, 1981 is redescribed and an update of its distribution in the North Sea is given. Stenocaris pygmaea NOODT, 1955, previously ranked as a junior subjective synonmy of S. pontica CHAPPUIS & SERBAN, 1953, is reinstated. The homology of spinous processes on the caudal rami in species of Boreopontia WILLEMS, Stenocaris SARS and Stenocaropsis APOSTOLOV is assessed.