Poitiers was founded by the Celtic Pictones tribe as the oppidum Lemonum before Roman influence. The name is said to have come from the Celtic word for elm, Lemo. After Roman influence, the town became known as Pictavium.
Until 1857 Poitiers contained the ruins of a vast Roman amphitheatre larger than that of Nîmes.
Remains of Roman baths, built in the 1st century and demolished in the 3rd century, were laid bare in 1877.
In 1879 a burial-place and tombs of a number of Christian martyrs were discovered on the heights to the south-east, the names of some of the Christians being preserved in paintings and inscriptions. Not far from these tombs is a huge dolmen (the Pierre Levée), which is 22 feet (6.7 m) long, 16 feet (4.9 m) broad and 7 feet (2.1 m) high, and around which used to be held the great fair of Saint Luke.
The Romans also built at least three aqueducts. This extensive ensemble of Roman constructions suggests Poitiers was a town of first importance, possibly even the capital of the Roman province of Gallia Aquitania during the 2nd century.
As Christianity was officialized and introduced across the Roman Empire during the 3rd and 4th centuries, the first bishop of Poitiers from 350 to 367, Saint Hilarius, evangelized the city. The first foundations of the Baptistère Saint-Jean are traced to that era.
In the 4th century, a thick wall six meters wide and ten meters high was built around the city. It was 2.5 km (1.6 mi) long and stood lower on the naturally defended east side and at the top of the promontory.
At this time, the town began to be known as Poitiers, after the original Pictones inhabitants.
Fifty years later the city fell into the hands of the Arian Visigoths, and became one of the principal residences of their kings. Visigoth King Alaric II was defeated by Clovis I at Vouillé, not far from Poitiers, in 507, and the town came under Frankish dominion