Uti possidetis is an expression that originated in Roman private law, where it was the name of a procedure used in litigation about land. It came from a praetorial edict that could be abbreviated "As you possess, so shall you possess". Later, by a misleading analogy, the phrase was transferred to international law, where it has had more than one meaning.

In Rome, if two parties disputed possession of land, the praetor preferred the one who was in actual occupation, unless he had got it from the other by force, stealth or as a temporary favour (nec vi, nec clam, nec precario). The contest was initiated by an interdict called uti possidetis. The winner was confirmed or restored in possession, and the loser was ordered not to displace him by force. However, the winner had not proved he was the real owner; only that, for the moment, he had a better right to possession than his opponent.

In the early modern era some European states, when dealing with other European states, used the phrase to justify the acquisition of territory by occupation. There was no universally agreed rule and, for example, Portugal applied it more ambitiously than Spain. Despite that, there is no doubt that important polities, such as Brazil, have been established on that interpretation. It was also a generally accepted rule about the interpretation of peace treaties. A peace treaty was presumed to give each party a permanent right to the territory it occupied at the conclusion of hostilities, unless the contrary was expressly stipulated. Whether this rule has survived in the international regime following the creation of the United Nations must be doubtful. This usage is sometimes called uti possidetis de facto.

In recent times uti possidetis refers to a doctrine for drawing international boundaries. When colonial territories achieve independence, or when a polity breaks up e.g. Yugoslavia, then, in default of a better rule, the old administrative boundaries between the new states ought to be followed. This doctrine, which has its critics, is sometimes called uti possidetis juris.