The Gaesatae or Gaesati (Greek Γαισάται) were a group of Gallic mercenary warriors who lived in the Alps near the river Rhône and fought against the Roman Republic in the Battle of Telamon of 225 BC.
According to some scholars, the Gaesatae may be identified with the Allobroges, who first appeared in the same region only a few years later in connection with Hannibal's crossing of the Alps in 218 BC.
It is cognate with Old Irish gaiscedach ('champion, armed person'), from gaisced ('weapons'), itself from gáe ('spear, javelin'). The Gaesatae have been compared with the medieval Irish fianna, who were small war-bands of landless young men operating independently of any kingdom.
According to Polybius' account, the Boii and Insubres of Cisalpine Gaul paid the Gaesatae, under their leaders Concolitanus and Aneroëstes, large sums of money to fight against the Romans, in response to the Roman colonisation of the former Gallic territory of Picenum. The Gauls overran and defeated a Roman army on the approach to Rome, but when the consul Lucius Aemilius Papus arrived with his troops, the Gauls followed Aneroëstes' advice to withdraw with their booty. Papus pursued them, and the other consul Gaius Atilius Regulus cut them off at Telamon in Etruria.
Polybius describes how the Gaesatae fought at the front, and unlike their Gallic allies who fought in trousers and light cloaks, they went into battle naked, both because of their great confidence and their desire not to get their clothes caught in the brambles. Diodorus Siculus also reports that some Gauls fought naked, trusting in the protection of nature. The appearance of these well-built naked warriors, and the noise of their trumpets and war-cries, intimidated the Romans, but their small shields offered little protection against Roman javelins, and the Gaesatae were driven back and their allies slaughtered. Concolitanus was captured. Aneroëstes escaped with a few followers and took his own life. In 222 BC the Gaesatae were hired again, but the Gallic forces were defeated by the Roman cavalry at Clastidium in the territory of the Insubres. According to Plutarch, in his Life of Marcellus, the Gaesatae numbered 30,000 as they crossed the Alps, of whom 10,000 fought at Clastidium.
- Kruta 2000, p. 290.
- Delamarre 2003, p. 174.
- MacKillop 2004, s.v. Gaesatae.
- Kruta 2000, p. 290: "Il s'agissait [les Gésates] de mercenaires transalpins appartenant à des populations qui étaient probablement installées alors sur la rive gauche du Rhône depuis quelques décennies seulement et semblent avoir été connues dans le dernier quart du me siècle av. J.-C., lors du passage d'Hannibal dans la région, sous le nom d'Allobroges (« gens d'un autre pays»)."
- Bocquet 2009, pp. 35–36.
- Polybius, Histories 2:22.1
- Dictionary of the Irish Language, Compact Edition, Royal Irish Academy, 1990, p. 352
- MacKillop 2004, s.v. Fianna: "An antecedent body may be the Gaulish gaesatae from the Upper Rhone as described by the Greek historian Polybius (2nd cent. bc ) ... Irish chronicles indicate that the first fianna were approximately contemporary with the gaesatae, as when they protected the ard rí [high king] Fiachach."
- Polybius, Histories 2.:5
- Polybius, Histories 2:26-27
- Polybius, Histories 2:28.3-7
- Diodorus Siculus, Historical Library 5.30
- Polybius, Histories 2:29.5-30.9
- Polybius, Histories 2:31.1-2
- Polybius, Histories 2.34; Plutarch, Marcellus 6-7
- Plutarch, Marcellus, chapters 6-7 
- Bocquet, Aimé (2009). Hannibal chez les Allobroges: 218 avant Jésus-Christ : la grande traversée des Alpes. La Fontaine de Siloë. ISBN 978-2-84206-419-8.
- Delamarre, Xavier (2003). Dictionnaire de la langue gauloise: Une approche linguistique du vieux-celtique continental. Errance. ISBN 9782877723695.
- Kruta, Venceslas (2000). Les Celtes, histoire et dictionnaire : des origines à la romanisation et au christianisme. Robert Laffont. ISBN 2-221-05690-6.
- MacKillop, James (2004). A dictionary of Celtic mythology. Oxford University Press. ISBN 0-19-860967-1.