Chalcedon was an ancient maritime town of Bithynia, in Asia Minor. It was located almost directly opposite Byzantium, south of Scutari and it is now a district of the city of Istanbul named Kadıköy. The name Chalcedon is a variant of Calchedon, found on all the coins of the town as well as in manuscripts of Herodotus's Histories, Xenophon's Hellenica, Arrian's Anabasis, and other works. Except for the Maiden's Tower, almost no above-ground vestiges of the ancient city survive in Kadıköy today; artifacts uncovered at Altıyol and other excavation sites are on display at the Istanbul Archaeological Museum.

Bithynia was an ancient region, kingdom and Roman province in the northwest of Asia Minor, adjoining the Sea of Marmara, the Bosporus, and the Black Sea. It bordered Mysia to the southwest, Paphlagonia to the northeast along the Pontic coast, and Phrygia to the southeast towards the interior of Asia Minor.