Bithynia was settled originally by the Thracians, around 550 BC, as an independent kingdom, but due to its location at the crossroads of Europe and Asia minor was always being fought over. It was first taken by the Lydians, then later incorporated into the Persian Empire by Cyrus the Great, where it still retained some autonomy.

After the death of Alexander the Great, the Bithynians were able to develop more or less independently from the Seleucids. Uniting under Ziboetes (aka Zipoites), Bithynia became a kingdom in its own right in when he was recognised as king c. 294 BC. Ziboetes I was succeeded (c. 280 B.C.) by his eldest son Nicomedes I, under whose rule Bithynia became increasingly Hellenised. In 264, Nicomedia was founded - still a major city during the Byzantine era.

During the reign of Nicomedes I and the subsequent reigns of Prusias I, Prusias II, and Nicomedes II, wars continued both with the Seleucids and with the rising power of Pergamon. In the 1st cent. B.C., Mithradates VI of Pontus had designs on Bithynia, which by then was ruled by Nicomedes IV (sometimes confused with Nicomedes III), a client of Rome. After being removed from and restored to the throne three times during the turbulent Mithridatic Wars, Nicomedes IV died childless in 75/74 BC, and bequeathed his country to Rome - maybe to ensure stability and protection for his people?

328-279 BC Ziboetes I

279-255 BC Nicomedes I opposed by...

279-276 BC Ziboetes II

255 BC Etazeta (regent with Ziboetes III)

255-228 BC Ziaelas

228-185 BC Prusias I Hol

185-149 BC Prusias II Kineg

149-128 BC Nicomedes II Epiphanes

128-94 BC Nicomedes III Euergetes

94-75 BC Nicomedes IV Philopater