Fort Koroibos was an Ancient Greek military fort located within the Valley of Olympia in Elis, Greece. Named for Koroibos of Elis, the first Olympic champion, the fort was garrisoned by Athenian troops at the time of the Olympic Games of 428 BC, which were held amid the Peloponnesian War. That same year, the mercenary Kassandra infiltrated the fort to seek the antidote for the slain Cult of Kosmos member Kallias' poison, which he had used to nearly kill Alcibiades, and Kassandra killed its polemarch and captains, burned its supplies, looted its valuables, and killed several mercenaries who had tracked her there.