In Roman law and legal traditions influenced by it, ius gentium or jus gentium (Latin for "law of nations" or "law of peoples") is the law that applies to all gentes ("peoples" or "nations"). It was an early form of international law, comprising not a body of statute law or legal code, but the customary law thought to be held in common by all in "reasoned compliance with standards of international conduct".

Ius gentium was regarded as a form of ius naturale, or natural law. Unlike ius civile, it applied to all persons and not only Roman citizens, as the rules of ius gentium could be derived from natural reason as innate in all of mankind.

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 what "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.

— Gaius, G. Inst. 1.1

As a form of natural law, the ius gentium was regarded as "innate in every human being", a view that was consonant with Stoic philosophy. Cicero distinguished between things that are written and those that are unwritten but upheld by the ius gentium or the mos maiorum, "ancestral custom". In his treatise De officiis, he regards the ius gentium as a higher law of moral obligation binding human beings beyond the requirements of civil law. A person driven into exile, for instance, lost his legal standing as a Roman citizen, but was supposed to retain the basic protections extended to all human beings under the ius gentium.

The 2nd-century Roman jurist Ulpian, however, divided law into three branches: natural law, which existed in nature and governed animals as well as humans; the law of nations, which was distinctively human; and, civil law, which was the body of laws specific to a people. Slavery, for instance, was supported by the ius gentium, even though under natural law all are born free (liberi). In this tripartite division of law, property rights might be considered a part of the ius gentium, but not of natural law. Hermogenianus, a Roman jurist of the second half of the 3rd century, described the ius gentium as comprising wars, national interests, kingship and sovereignty, rights of ownership, property boundaries, settlements, and commerce, "including contracts of buying and selling and letting and hiring, except for certain contractual elements distinguished through ius civile". The ius gentium was thus in practice important in facilitating commercial law.