Tium (Greek: Τῖον) was an ancient settlement, also known as Filyos (Greek: Φίλειος), on the south coast of the Black Sea at the mouth of the river Billaeus in present-day Turkey. Ancient writers variously assigned it to ancient Paphlagonia or Bithynia.
The town was founded as a colony from the Greek city of Miletus in the 7th century BCE. According to Strabo, the town was only remarkable as the birthplace of Philetaerus, founder of the royal dynasty of Pergamon. At the beginning of the 3rd century BCE, Amastrine (Amastris), the niece of the last Persian king Darius III, who was the wife of Dionysius, tyrant of Heracleia, and after his death the wife of Lysimachus caused a synoecism of Sesamus, Cytorus, Cromna, all towns mentioned in the Iliad, and Tium after her separation from Lysimachus, to form the new community of Amastris. Tium, says Strabo, soon detached itself from the community, but the rest kept together, probably in 282 BCE, recovered its autonomous status.
Tium was part of Kingdom of Bithynia, which on the death of King Nicomedes IV in 74 BC became a Roman province. Emperor Theodosius I (379–392) incorporated it into Honorias, when he carved out this new province from portions of Bithynia and Paphlagonia and named it after his younger son Honorius. In 535, the Emperor Justinian united Honorias with Paphlagonia in a decree that expressly mentioned Tium among the cities that were affected. There are coins of Tium as late as the reign of Gallienus, on which the ethnic name appears as Τιανοί, Τεῖοι, and Τειανοί.
This see figures in all the Notitiae episcopatuum.