The eastern Armorican people of Redones founded Condate— an ancient Celtic word meaning confluent— at the confluence of the Ille and Vilaine rivers and made it the capital of a territory that extended to the Bay of Mont Saint-Michel. The name of the city of Rédon also reflects that of the Redones. Early in the 1st century BCE, they adopted the Greek and Roman practice of issuing coinage , adapting the widely-imitated gold staters of Philip II of Macedon, in the characteristic Celtic coin metal alloy called billion. Without inscriptions, as the Celtic practice was, the Redones coinage features a carioteer whose pony has a human head. Large hoards of their coins were unearthed in the "treasure of Amanlis" found in June 1835 and that of Saint-Jacques-de-la-Lande, discovered in February 1941. The Museum at Rennes contains a large representative collection.
They joined the Gaulish coalition against Rome in 57 BCE, which was suppressed by Crassus. The following year, Roman emissaries were held hostage by the Redones, which obliged Julius Caesar to intervene in Armorica and suppress the rebels, and the following year to cross the Channel to discourage further support of the Redones by the Britons. In 52 the Redones responded to the call of Vercingetorix to furnish a large contingent of warriors (Caesar, Gallic Wars II.34; VII.75).
In the Roman era, Condate became Condate Riedonum, capital of civitas riedonum
The oldest known rennais is Titus Flavius Postuminus, known to us from his steles found in Rennes in 1968. As indicated by his name, he would have been born under the Flavian dynasty, under the reign of Titus, i.e. between 79 and 81 AD. One of the steles tell us, in Latin, that he took charge over all the public affairs in the Civitas Riedonum. He was twice duumvir and flamine for life for Mars Mullo.
During the Roman era, the strategic position of the town contributed to its importance. To the west the principal Roman route, via Osismii stretched from Condate to Vorgium (modern Carhaix).
In the year 275, the threat of barbarians led to the erection of a robust brick wall around Rennes. Rennes became known as the "red town".
Once threatened by the danger of bagaudes at the end of the Roman Empire in the 5th century, the Armorican peninsula, including Brittany and therefore Rennes, made up the last of the stronghold of the western Roman Empire. The invincible Armorican Romans held their ground against Clovis I, who occupied most of Alamans, then the Visigoths. Melaine, the bishop of Rennes, played an important role in the peace treaty between the Franks and the Armoricans in the year 497. He famously declared "Il faut faire la paix entre chrétiens" ("Peace must be made between Christians").
Starting in the fifth century, Bretons occupied the western part of the Armorican penensula, which started to be called little Britain, and then Brittany, while the Franks took the rest of Armorica. To contain the expansion and avoid Breton incursions, the Carolingians instituted a Breton march, composed of the counties of Rennes, Nantes, and Vannes.
These marches were entirely absorbed by the Breton Kingdom in the ninth century, and Rennes became Breton in 851. Rennes would later become the capital of Ducal Brittany.
During the Breton War of Succession, in 1356 and 1357, the city was sieged by the Duke of Lancaster, brother of the English king, but Bertrand du Guesclin slipped into the city and took over the resistance, which would ultimately be victorious. After nearly a year, Lancaster renounced the English siege in 1357.
In 1491, it was the French army of Charles VIII, led by his general, La Trémoïlle, that unsuccessfully attacked Rennes. Brittany having already capitulated elsewhere, Rennes alone still resisted. The defenders of Rennes were determined to resist to the death, but the Duchess Anne of Brittany chose instead to negotiate. By her marriage to Charles VIII, she made Brittany a part of France. Anne jealously guarded Brittany's autonomy, but the duchy was eventually fully merged with the French crown by her daughter Claude of France.