Astacus /ˈæstəkəs/ is the name of an ancient city in Bithynia; it was also called Olbia /ˈɒlbiə/. Stephanus of Byzantium records an aetiological myth that it was founded by Astacus, son of Poseidon and the nymph Olbia.
The traditional date of the founding is 712/11 BC, the first year of the 17th Olympiad. In the second book of his History of the Peloponnesian War Thucydides mentions that the tyrant Evarchus was ruler of the town for a period during the years of the decade 430 to 420 BC.
King Zipoetes I of Bithynia made two attempts to absorb Astacus into his kingdom: in 315 BC he was defeated by succors sent by Antigonus Monophthalmos. In 301 BC, he was successful, but the city was destroyed in the war.
Nicomedes I, son of Zipoetes, founded a new city to replace Astacus across from its former location, which he named Nicomedia after himself, bringing some of the Astacan cults to the new site. Nicomedia remained the capital of Bithynia, and became one of the great cities of the Roman east; the Emperor Diocletian made it his usual capital.