Re-examination of the type material of Ismardis spartacus Leigh-Sharpe, 1936 (Copepoda, Harpacticoida), discovered in the gonad and mantle cavity of Patella caerulea L. (Mollusca), revealed that the species should be allocated to the genus Harpacticus Milne-Edwards, 1840, and not to the Thalestridae as previously suggested. The genus Ismardis and the family Ismardiidae are relegated to junior synonyms of Harpacticus and Harpacticidae, respectively. Harpacticus spartacus, new combination, is conspecific with the “schwächeren” form of the Mediterranean H. nicaeensis Claus, 1866. The taxonomic confusion surrounding H. gracilis Claus, 1866 is reviewed, and the species is regarded as unrecognizable, rendering all subsequent records unconfirmed, doubtful, or erroneous. At least some material previously identified with this species proved to be conspecific with H. longiantennata Apostolov and Petkovski, 1980 (Banyuls-sur-Mer) or based on an amalgamate of H. obscurus T. Scott, 1895, and H. giesbrechti Klie, 1927 (Norway). Several species formerly relegated to junior synonyms of H. gracilis are reinstated as species inquirendae: H. elongatus Boeck, 1865; H. dentatus Krichagin, 1873; and H. fucicolus T. Scott, 1912. The only European record of H. pulvinatus Brady, 1910, being otherwise endemic to the Southern Hemisphere, is based on a misidentification, being attributable to H. nicaeensis. Other records of the latter from outside the Ponto-Mediterranean basin are unverifiable and probably false, including the Angolese material of H. nicaeensis, which is regarded as species inquirenda in Harpacticus.