The Council of Chalcedon (; Latin: Concilium Chalcedonense; Greek: , Synodos ts Chalkdonos) was the fourth ecumenical council of the Christian Church. It was convoked by the Roman emperor Marcian. The council convened in the city of Chalcedon, Bithynia (modern-day Kadikoy, Istanbul, Turkey) from 8 October to 1 November 451 AD. The council was attended by 520 bishops or their representatives, making it the largest and best-documented of the first seven ecumenical councils. The principal purpose of the council was to re-assert the teachings of the ecumenical Council of Ephesus against the heresies of Eutyches and Nestorius. Such heresies attempted to dismantle and separate Christ's divine nature from his humanity (Nestorianism) and further, to limit Christ as solely divine in nature (Monophysitism).

Chalcedon ( or ; Greek: Χαλκηδών, sometimes transliterated as Chalkedon) was an ancient maritime town of Bithynia, in Asia Minor. It was located almost directly opposite Byzantium, south of Scutari (modern Üsküdar) 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.

The site of Chalcedon is located on a small peninsula on the north coast of the Sea of Marmara, near the mouth of the Bosphorus. A stream, called the Chalcis or Chalcedon in antiquity and now known as the Kurbağalıdere (Turkish: stream with frogs), flows into Fenerbahçe Bay. There, Greek colonists from Megara in Attica founded the settlement of Chalcedon in 685 BC, some seventeen years before Byzantium.

The Greek name of the ancient town is from its Phoenician name qart-ħadaʃt, meaning "New Town", whence Karkhēd(ōn), as similarly is the name of Carthage. The mineral chalcedony is named after the city.