Helenopolis (Greek: Ἑλενόπολις) or Drepana (Δρέπανα) was an ancient Roman and Byzantine town in Bithynia, Asia Minor, on the southern side of the Gulf of Astacus. It has been identified with the modern village of Hersek, in the district of Altınova, Yalova Province. It is traditionally considered as the birthplace of Saint Helena.
According to the 6th-century historian Procopius, Helena's son Constantine the Great renamed the city "Helenopolis" to honor her birthplace; but the name may simply have honored her without marking her birthplace. Constantine also built there a church in honour of the martyr Saint Lucian; it soon grew in importance, and Constantine lived there very often towards the end of his life.
Near it were some famous mineral springs. These mineral springs might be those of Termal near Yalova.
Justinian built there an aqueduct, baths, and other monuments. It does not seem ever to have grown, and it was slightingly called Eleinou Polis, "the wretched town".