Chalcedon (Χαλκεδον, sometimes transliterated by purists as Chalkedon) was an ancient maritime town of Bithynia, in Asia Minor, almost directly opposite Byzantium, south of Scutari.

It was a Megarian colony founded on a site so obviously inferior to that which was within view on the opposite shore, that it received from the oracle the name of "the City of the Blind."

In its early history it shared the fortunes of Byzantium, was taken by the satrap Otanes, vacillated long between the Lacedaemonian and the Athenian interests, and was at last bequeathed to the Romans by Attalus III of Pergamum (133 b.c.).

It was partly destroyed by Mithradates, but recovered during the Empire, and in AD. 451 was the location of the Council of Chalcedon.

It fell under the repeated attacks of the barbarian hordes who crossed over after having ravaged Byzantium, and furnished an encampment to the Persians under Chosroes, c. 616-626. The Turks used it as a quarry for building materials for Constantinople.

To the south are the ruins of Panteichion (mod. Pendik), where Belisarius is said to have lived in retirement.

The name of the mineral chalcedony is derived from that of this town.

It is today Kadıköy, a district of Istanbul, Turkey.