The Gulf of Trieste (Italian: Golfo di Trieste, Slovene: Tržaški zaliv, Croatian: Tršćanski zaljev, German: Golf von Triest) is a shallow bay of the Adriatic Sea, in the extreme northern part of the Mediterranean Sea. It is part of the Gulf of Venice and is shared by Italy, Slovenia and Croatia. It is closed to the south by the peninsula of Istria, the largest peninsula in the Adriatic Sea, shared between Croatia and Slovenia.

The gulf is limited by an imaginary line connecting the Punta Tagliamento on the Italian and Savudrija (Punta Salvore) on the Croatian coast. Its area is approximately 550 km², its average depth 16 m, and its maximum depth 37 m. With the exception of flat islets blocking the entrance to Laguna di Grado, there are no islands in the gulf. Its eastern coasts, with Trieste and the Slovenian Littoral, have more rugged relief.

The sea current in the gulf flows counterclockwise. Its average speed is 0.8 knots. Tides in the gulf are among the largest in the Adriatic Sea, but nevertheless do not usually exceed 60 centimetres (24 in). The average salinity is 37-38‰, but in the summer it falls under 35‰.