Roman province of northwestern Asia Minor, originally containing just the penninsula of the cities of Chalcedon and Nicomedia, on the other side of the strait from Byzantium, but later extended eastwards to include Pontus, towards the borders of Armenia.
The recorded history of the region begins in the middle of the 6th century BC, when Greek colonists hoping to exploit the trade routes and fertile grain-field of the lands bordering the Black Sea encountered kingdoms of 'Asiatic Thracians'. This Second Ionia, mostly mountainous, nevertheless held rich, fertile plains fed by the Sangarius river. Thick tree-growth and quarries provided lumber and marble for Ionia, Greece, Syria, and, later, Rome, shipped along the rivers or the lanes of the Pontus or brought by caravan along some of the well-developed roads binding the regions together.
Politically, Bithynia maintained autonomy for a long period, though during the Seleucid dynasties it donated both tribute and mercenaries to the Persian armies. Otherwise, it took advantage of the Armenian buffer states to the east and Greek trading colonies to the west to develop a strong mercantile economy.
Rome first became involved with Bithynia at the end of the 2nd century, when the traders and money-lenders of an ever-expanding Roman empire began taking advantage of its economic wealth. When C. Marius appealed to king Nicomedes III to lend him mercenaries to fight the Gallic Cimbrians, the king complained that Roman ships had already clapped too many Bithynians into chains, leading to a senatorial debate ending in a decree that all foreign non-Romans born free were to be freed. These unofficial merchant interests in the Black Sea soon drew the Roman government into the fray between local political rivals; envoys worked actively to prevent a resolution between Mithridates VI of Pontus and Nicomedes to settle the ownership of lands between the kingdoms.
The reason for the interference soon became clear when, after the death of Nicomedes III, when his son, Nicomedes IV took the throne, Mithridates conquered the kingdom. Rome used the opportunity to send an army under Sulla to retake the land and restore Nicomedes IV, but by the end of the war, called the First Mithridatic War, the king was deeply in debt and heavily reliant on the new Roman aid. When he finally died in 75 BC, he left his entire nation to the republic of Rome.
For the next century, Rome worked furiously to bring the impoverished Bithynia back to prosperity. As usual, Roman Publicani, private citizens who payed local governors for the privilege of collecting taxes (and thus naturally bled the citizens dry to cover their expense and make a profit) moved in and continued to exploit the situation, leading to increased economic troubles. It is no wonder that Catullus, in his brief tenure as an aid to the proconsul C. Memmius, complains about the extreme poverty of the land, and his inability earn his riches there. But by the time of the empire, thanks in no small part to the abundant natural resources, Bithynia had again become one of the richest provinces belonging to Rome; Tacitus speaks of the unbridled success of Petronius Arbiter as proconsul there. Indeed, as a trade center, it became so important that Trajan and Hadrian assigned much of the administration to personally picked legates, an arrangement made permanent under Marcus Aurelius.
Bithynia also offers some of the richest evidence for the local society and administration of the provinces, thanks to the letters of Pliny the Younger, proconsul there in AD 108. One of his letters (X.96) speaks at length of the problems he had been having with the cults of the Christians, admitting that, while he knew next to nothing of their beliefs, they should clearly be executed for their disobedience and "detestable superstition". Interestingly enough, he also refuses to hunt them out actively, saying that he would exact punishment only when they had already been arrested for other crimes or made a public nuisance of themselves.