The ius gentium or jus gentium (Latin, "law of nations") is a concept of international law within the ancient Roman legal system and the Western law traditions based on or influenced by it. The ius gentium is not a body of statute law or a legal code, but rather customary law thought to be held in common by all gentes ("peoples" or "nations") in "reasoned compliance with standards of international conduct."

Following the Christianization of the Roman Empire, canon law also contributed to the European ius gentium. By the 16th century, the shared concept of the ius gentium disintegrated as individual European nations developed distinct bodies of law, the authority of the Pope declined, and colonialism created subject nations outside the West.

In classical antiquity, the ius gentium was regarded as an aspect of natural law (ius naturale), as distinguished from civil law (ius civile). The jurist Gaius defined the ius gentium as that which "natural reason has established among all peoples":

Every people (populus) that is governed by statutes and customs (leges et mores) observes partly its own peculiar law and partly the common law of all mankind. That law which a people established for itself is peculiar to it and is called ius civile (civil law) as being the special law of that civitas (state), while the law that natural reason establishes among all mankind is followed by all peoples alike, and is called ius gentium (law of nations, or law of the world) as being the law observed by all mankind. Thus the Roman people observes partly its own peculiar law and partly the common law of all mankind.