The Gulf of Trieste is a shallow bay of the Adriatic Sea, in the extreme northern part of the Adriatic 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 entire Slovenian sea is part of the Gulf of Trieste.

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 square kilometres (210 sq mi), its average depth is 18.7 metres (61 ft), and its maximum depth is 37 metres (121 ft). With the exception of flat islets blocking the entrance to Marano-Grado lagoon, 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 below 35‰.

Its most prominent features are:

• The Bay of Panzano in Italy
• The Bay of Muggia in Italy
• The Bay of Grignano in Italy
• The Bay of Koper in Slovenia
• The Gulf of Piran, the sovereignty over which has been a matter of dispute between Croatia and Slovenia since 1991.

The entire Slovenian coastline is located on the Gulf of Trieste. Its length is 46.6 kilometres (29.0 mi). Towns along the coastline include (from east to west) Koper, Izola, and Piran.

• Slovenian Riviera
• Free Territory of Trieste
  • Treaty of Osimo
• Barcolana regatta

• Media related to Gulf of Trieste at Wikimedia Commons
• Gulf of Trieste on Geopedia.si Archived 2016-03-04 at the Wayback Machine (map, relief, orthophoto)
• Conditions in the Gulf of Trieste on and near the Slovene coast:
  • Koper - graphs, in the following order, of water level and temperature data for the past 30 days (taken in Koper by ARSO)
  • Piran - graphs, in the following order, of water temperature, wave height, wave (interval) period, wave direction, current speed, current direction, maximum wave height data for the past 30 days (taken near Piran by ARSO)