An international strait is a narrow natural waterway connecting two parts of the high seas or exclusive economic zones, used for international navigation. Per the United Nations Convention on the Law of the Sea (UNCLOS), a transit passage regime prevails in such straits for both ships and aircraft with few exceptions, even when the territorial waters of bordering country or countries overlap. Worldwide, more than 200 straits might satisfy the criteria of an international strait. Notable international straits include the Bosporus and Dardanelles, Strait of Magellan, Strait of Gibraltar, Strait of Dover, Danish Straits and the Strait of Hormuz.

The term is defined in articles 37 and 38 of the UNCLOS III. The convention does not use the words "international strait" to prevent a confusion with international waters,^{[citation needed]} describing instead the straits used for international navigation.

Some experts suggest a broader definition of the "international strait". In particular, the following list is proposed:

The interest in the rights of navigation through straits dates as far back as the Peloponnesian War (c. 422 BC). Grotius early in the 17th century recognized the right of the owner of the shore to appropriate the water expanses that can be seen from the shore, and an obligation of the owner to allow innocent (unarmed) navigation through these waters. Grotius also suggested the right for the owner to collect fees for such passage. The latter practice, supported by Puffendorf and later by de Vattel, persisted until the mid-19th century, when Denmark dropped the levies for the passage of Danish belts and sunds (cf. the Copenhagen Convention of 1857 abolishing the Sound Dues that were collected for four hundred years).

De Vattel was the first to highlight the difference between the straits that "serve as a mean of communication between two [high] seas" and the one without such function. The passage of ships through the former cannot be prohibited as long as it does not adversely affect the security of the coastal state. While Vattel based his ideas on the Roman law concept of right of way, many other 18th century thinkers supported the right of a coastal state to exclude foreign ships from its territorial waters.

With the arrival of steamships in the 19th century, the ability to navigate along the shortest route became a significant economic factor, spurring development of special regimes for transit, similar to high seas. At the same time, a distinction was made between the wartime and peacetime navigation.

A major effort on codification of the legal regime for the straits was made by the Institut de Droit International (IDI) between 1894 and 1912. 1894, in particular, saw the proclamation of the innocent passage principle in those straits where there are no high seas lanes due to overlapping territorial waters. Some work was also done by the International Law Association (ILA) between 1893 and 1910 and Inter-Parliamentary Union between 1910 and 1915. Like the IDI's declarations, these efforts were stopped by the First World War, no international agreements followed, and the discussion at the Second Peace Conference (1907) only made it clear that the international straits require their own regime. No agreement was reached at the 1930 Hague Conference on Codification either. Overall, per the 1992 statement of the Office for Ocean Affairs and the Law of the Sea, "the treatment of the question of straits [...] between 1894 and 1930 was unsatisfactory".

The "now classic" book on international straits by Erik Brüel, International Straits. A Treatise on International Law, was published in the 1947.