The Golden Horn from the southern or Constantinople shore, with the skyline of modern Istanbul on the far shore

The Golden Horn (in Turkish Haliç, in Greek Khrysokeras or Chrysoceras or Χρυσοκερας) is an estuary dividing the city of Istanbul.

With the Sea of Marmara, the Golden Horn forms a peninsula with a deep natural harbour. This site was originally settled by ancient Greek colonists as the city of Byzantium. The Byzantine Empire had its naval headquarters there, and walls were built along the shoreline to protect the city (by then renamed Constantinople) from naval attacks. At the entrance to the Horn, there was a large, floating boom-chain (a huge line of logs bound together by chains) pulled across from Constantinople to the fortress of Galata on the northern side, preventing unwanted ships from entering.

There were three notable times when the chain across the Horn was either broken or circumvented. In the 10th century the Kievan Rus' dragged their ships out of the Bosporus, around Galata, and relaunched them in the Horn; the Byzantines defeated them with Greek fire. In 1204, during the Fourth Crusade, Venetian ships were able to break the chain with a ram. In 1453, Ottoman Sultan Mehmed II copied the tactics of the Rus', towing his ships across Galata into the estuary over greased logs.

In 1502 Leonardo da Vinci produced a drawing of a single span 720-foot (240 m) bridge as part of a civil engineering project for Sultan Beyazid II. The bridge was intended to span the Golden Horn. It was never built, but Leonardo's vision was resurrected in 2001 when a smaller bridge based on his design was constructed in Norway.

After the Fall of Constantinople to Mehmed, Greek citizens, the Greek Orthodox Church, Jews, Italian merchants, and other non-Muslims began to live along the Horn in the Phanar (Fanar) and Balat districts. Today the Golden Horn is settled on both sides, and there are parks along each shore. The Istanbul Chamber of Commerce is also located along the shore, as is a Muslim cemetery. The Galata Bridge, built in 1836, connects Old Istanbul with the districts of Galata and Pera (Beyoglu). Two other bridges, the Atatürk Bridge and the Haliç Bridge, are located further up the Horn. Until the 1980s the Horn was a dumping ground for industrial waste, but has since been cleaned up and is a popular tourist attraction in Istanbul.