The United Nations Convention on the Law of the Sea (UNCLOS), also called the Law of the Sea Convention or the Law of the Sea Treaty, is an international agreement that establishes a legal framework for all marine and maritime activities. As of June 2016, 167 countries and the European Union are parties.

The convention resulted from the third United Nations Conference on the Law of the Sea (UNCLOS III), which took place between 1973 and 1982. UNCLOS replaced the four treaties of the 1958 Convention on the High Seas. UNCLOS came into force in 1994, a year after Guyana became the 60th nation to ratify the treaty.[1] In 2023, agreement was reached on a High Seas Treaty to be added as an instrument of the convention, to protect ocean life in international waters. This would provide measures including Marine Protected Areas and environmental impact assessments.

While the secretary-general of the United Nations receives instruments of ratification and accession and the UN provides support for meetings of states party to the convention, the United Nations Secretariat has no direct operational role in the implementation of the convention. A UN specialized agency, the International Maritime Organization, does play a role, however, as well as other bodies such as the International Whaling C and the International Seabed Authority (ISA), which was established by the convention itself.

UNCLOS replaces the older "freedom of the seas" concept, dating from the 17th century. According to this concept, national rights were limited to a specified belt of water extending from a nation's coastlines, usually 3 nautical miles (5.6 km; 3.5 mi) (three-mile limit), according to the "cannon shot" rule developed by the Dutch jurist Cornelius van Bynkershoek. All waters beyond national boundaries were considered international waters: free to all nations, but belonging to none of them (the mare liberum principle propounded by Hugo Grotius).

In the early 20th century, some nations expressed their desire to extend national claims: to include mineral resources, to protect fish stocks, and to provide the means to enforce pollution controls. (The League of Nations called a 1930 conference at The Hague, but no agreements resulted.) Using the customary international law principle of a nation's right to protect its natural resources, President Harry S. Truman in 1945 extended United States control to all the natural resources of its continental shelf. Other nations were quick to follow suit. Between 1946 and 1950, Chile, Peru, and Ecuador extended their rights to a distance of 200 nautical miles (370 km; 230 mi) to cover their Humboldt Current fishing grounds. Other nations extended their territorial seas to 12 nautical miles (22 km; 14 mi).

By 1967, only 25 nations still used the old three nautical mile limit,[8] while 66 nations had set a 12-nautical-mile (22 km) territorial limit and eight had set a 200-nautical-mile (370 km) limit. As of 15 July 2011, only Jordan still uses the 3-mile (4.8 km) limit. That limit is also used in certain Australian islands, an area of Belize, some Japanese straits, certain areas of Papua New Guinea, and a few British Overseas Territories, such as Gibraltar.

UNCLOS does not deal with matters of territorial disputes or to resolve issues of sovereignty, as that field is governed by rules of customary international law on the acquisition and loss of territory.