Bithynia

Bithynia is a historical region in the northwest of ancient Anatolia, corresponding roughly to parts of present‑day northwestern Turkey, including areas around Bursa, Izmit, Iznik, and Bolu. It lies along the Propontis (Sea of Marmara) and extends inland toward the interior, with coastal cities on the Marmara and Black Sea littorals. Its heartland was a mix of coastal plains and uplands.

In antiquity Bithynia developed as a Hellenized polity. It emerged as a kingdom under rulers such as Zipoetes I and Nicomedes I (late 4th to 3rd century BCE), followed by subsequent kings including Prusias I, II and III. The kingdom maintained strong urban centers, minted coinage, and engaged in alliances and hostilities with neighboring Hellenistic states and Rome. In 74 BCE, after the death of Nicomedes IV, Rome annexed Bithynia, forming the province of Bithynia et Pontus.

Under Roman rule, the region contained important cities such as Nicaea (İznik), Nicomedia (İzmit), Prusa (Bursa), and Heraclea Pontica (on the Black Sea coast). Nicaea hosted the First Council of Nicaea in 325 CE, a defining event in early Christian history. In late antiquity, Bithynia remained part of the Eastern Roman (Byzantine) realm and continued to be an administrative and religious center until the medieval period, after which its western portions were absorbed into successive states, culminating in the Ottoman era. The historical region roughly corresponds to portions of modern Turkish provinces and remains a key reference point in classical geography and early Christian studies.