Chalcedon, (modern English pronunciation /kælˈsiːdən/ or /ˈkælsᵻˌdɒn/; Greek: Χαλκηδών),  was a maritime city in Bithynia on the Asian shore of the Bosporus across from ancient Constantinople. A major church in Chalcedon was the site of the Fourth Ecumenical Council in 451. The city is now called Kadikoy.
Chalcedon is located in Turkey on a small peninsula on the north coast of the Sea of Marmara, near the mouth of the Bosporus. While archaeological evidence has been found tracing settlements on the Asian side of the Bosporus back to the Chalcolithic period of 5500 to 3500 years before Christ, the first settlement by Greeks, from Megara in Attica, was established about 685/676 BC, several years before the founding of Byzantium on the other side of the straits.
While viewed as the ‘City of the Blind’ in antiquity because the site of Chalcedon was so inferior to that of Byzantium, with its Golden Horn harbor, that could be seen across the straits,  Chalcedon flourished as a trading city in the Phoenician trading routes.
Through the following centuries, the town shared its fortunes with Byzantium as it saw the passing of the Persians under Darius and allied itself at times with Athens and Sparta. In the first century before Christ it came into the hands of the Romans when king Nicomedes of Bithynia willed the area to them upon his death. Overshadowed by the proximity of Constantinople, as Byzantium was renamed after Constantine I’s death, the city was often used as a source of building stone for the monuments of the Eastern Roman capital.
In 361, Chalcedon was the site of the tribunal of Julian the Apostate where he brought his enemies to trial. The city was a see that became famous as the site of the Fourth Ecumenical Council in 451. The council was held at a magnificent church that was situated of the hill at Haider Pasha. This church was later destroyed by Suleiman when he had his mosque built in Constantinople.
In the seventh century, Chalcedon was held captive by the Persian forces of Chosroes II and those of the Arabs of Yazd.
After the fourth council the see was raised to the dignity of metropolis. With the fall of Constantinople to the Latins in 1204, the city became a Latin see that was suffragan of Nicomedia. Chalcedon fell under the control of the Ottoman Turk in 1454.
Today Chalcedon is a Turkish district known as Kadıköy, a part of cosmopolitan Istanbul. The Metropolis of Chalcedon is a jurisdiction within the Church of Constantinople in which the Cathedral of St. Euphemia is the seat of Metropolitan Athanasios (Papas).
A number of martyrs are associated with Chalcedon. These include:
- The virgin St. Euphemia and her companions in the early fourth century.
- St. Sabel the Persian and his companions.
- St. Adrian the martyr.