Chalcedon (/ˈkælsɪˌdɒn, kælˈsiːdən/; Ancient Greek: Χαλκηδών, romanized: Khalkēdṓn) was an ancient maritime town of Bithynia, in Asia Minor, Turkey. 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 are the names of Carthage and Cartagena. The mineral chalcedony is named after the city.

The mound of Fikirtepe has yielded remains dating to the Chalcolithic period (5500–3500 BC) and attest to a continuous settlement since prehistoric times. Phoenicians were active traders in this area.

Pliny states that Chalcedon was first named Procerastis, a name which may be derived from a point of land near it: then it was named Colpusa, from the harbour probably; and finally Caecorum Oppidum, or the town of the blind.

Chalcedon originated as a Megarian colony in 685 BC. The colonists from Megara settled on a site that was viewed in antiquity as so obviously inferior to that visible on the opposite shore of the Bosphorus (with its small settlements of Lygos and Semistra on Seraglio Point), that the 6th-century BC Persian general Megabazus allegedly remarked that Chalcedon's founders must have been blind. Indeed, Strabo and Pliny relate that the oracle of Apollo told the Athenians and Megarians who founded Byzantium in 657 BC to build their city "opposite to the blind", and that they interpreted "the blind" to mean Chalcedon, the "City of the Blind".

Nevertheless, trade thrived in Chalcedon; the town flourished and built many temples, including one to Apollo, which had an oracle. Chalcedonia, the territory dependent upon Chalcedon, stretched up the Anatolian shore of the Bosphorus at least as far as the temple of Zeus Urius, now the site of Yoros Castle, and may have included the north shore of the Bay of Astacus which extends towards Nicomedia. Important villages in Chalcedonia included Chrysopolis (the modern Üsküdar) and Panteicheion (Pendik). Strabo notes that "a little above the sea" in Chalcedonia lies "the fountain Azaritia, which contains small crocodiles".

In its early history Chalcedon shared the fortunes of Byzantium. Later, the 6th-century BC Persian satrap Otanes captured it. The city vacillated for a long while between the Lacedaemonian and the Athenian interests. Darius the Great's bridge of boats, built in 512 BC for his Scythian campaign, extended from Chalcedonia to Thrace. Chalcedon formed a part of the kingdom of Bithynia, whose king Nicomedes willed Bithynia to the Romans upon his death in 74 BC.