Foederati (/ˌfɛdəˈreɪtaɪ/ FED-ə-RAY-ty; singular: foederatus /ˌfɛdəˈreɪtəs/ FED-ə-RAY-təs) were peoples and cities bound by a treaty, known as foedus, with Rome. During the Roman Republic, the term identified the socii, but during the Roman Empire, it was used to describe foreign states, client kingdoms or barbarian tribes to which the empire provided benefits in exchange for military assistance. The term was also used, especially under the empire, for groups of barbarian mercenaries of various sizes who were typically allowed to settle within the empire.

In the early Roman Republic, foederati were tribes that were bound by a treaty (foedus /ˈfiːdəs/) to come to the defence of Rome but were neither Roman colonies nor beneficiaries of Roman citizenship (civitas). Members of the Latini tribe were considered blood allies, but the rest were federates or socii. The friction between the treaty obligations without the corresponding benefits of Romanity led to the Social War between the Romans, with a few close allies, and the disaffected socii. A law of 90 BC (Lex Julia) offered Roman citizenship to the federate states that accepted the terms. Not all cities were prepared to be absorbed into the Roman res publica (Heraclea and Naples). Other foederati lay outside Roman Italy such as Gades (Cádiz) and Massilia (Marseille).^{[clarification needed]}

The term foederati had its usage and meaning extended by the Romans' practice of subsidising entire barbarian tribes such as the Franks, Vandals, Alans, Huns and the Visigoths, the last being the best known, in exchange for providing warriors to fight in the Roman armies. Alaric I began his career leading a band of Gothic foederati.

At first, the Roman subsidy took the form of money or food, but as tax revenues dwindled in the 4th and the 5th centuries, the foederati were billeted on local landowners, which became identical to being allowed to settle on Roman territory. Large local landowners living in distant border provinces (see "marches") on extensive villas, which were largely self-sufficient, found their loyalties to the central authority, which were already conflicted by other developments, further compromised in such situations. As loyalties wavered and became more local, the empire then began to devolve into smaller territories and closer personal fealties.

The first Roman treaty with the Goths was after the defeat of Ariaric in 332, but whether or not it was a foedus is unclear.

The Franks became foederati in 358, when Emperor Julian let them keep the areas in northern Roman Gaul, which had been depopulated during the preceding century. Roman soldiers defended the Rhine and had major armies 100 miles (160 km) south and west of the Rhine. Frankish settlers were established in the areas north and east of the Romans and helped the Roman defence by providing intelligence and a buffer state. The breach of the Rhine borders in the frozen winter of 406 and 407 ended the Roman presence along the Rhine when both the Romans and the allied Franks were overrun by a massive tribal migration of Vandals and Alans.

In 376, some of the Goths asked Emperor Valens to allow them to settle on the southern bank of the Danube River and were accepted into the empire as foederati. The same Goths then revolted in retaliation for abuses and defeated the Romans in the Battle of Adrianople in 378. The critical loss of military manpower thereafter forced the Empire to rely much more on foederati levies.

The loyalty of the tribes and their chieftains was never reliable, and in 395, the Visigoths, now under the lead of Alaric, once again rose in rebellion. The father of one of the most powerful late Roman generals, Stilicho, rose from the ranks of the foederati.