The Golden Horn (Turkish: Haliç (which is derived from the Arabic word Khaleej, meaning Gulf) or Altın Boynuz (literally "Golden Horn" in Turkish); Greek: Κεράτιος Κόλπος, Keratios Kolpos) is a historic inlet of the Bosphorus dividing the city of Istanbul and forming the natural harbor that has sheltered Greek, Roman, Byzantine, Ottoman and other ships for thousands of years. It is a scimitar-shaped estuary that joins the Bosphorus just at the point where that strait enters the Sea of Marmara, thus forming a peninsula the tip of which is "Old Istanbul" (ancient Byzantion and Constantinople). Its Greek and English names mean the same, but the significance of the designation "golden" is obscure, while its Turkish name Haliç simply means "estuary". It has witnessed many tumultuous historical incidents and its dramatic vistas have been the subject of countless works of art.

The Golden Horn is a flooded prehistoric estuary. It is 7.5 kilometers long and, at its widest, 750 meters across. Its maximum depth, where it flows into the Bosphorus, is about 35 meters. It is today spanned by four bridges. They are, moving downstream, the Haliç Bridge (literally Estuary Bridge); the Eski Galata Bridge (literally Old Galata Bridge, as the former Galata Bridge was moved here in pieces, re-assembled and restored after a fire in 1992 damaged it; the current Galata Bridge which replaced it was completed in 1994); the Atatürk (Unkapanı) Bridge; and the Galata Bridge. A fifth bridge is currently under construction to connect the subway lines of the Istanbul Metro to the north and south of the Golden Horn.