This is another British tribe that shares a name with a tribe in pre-Roman France. They were the second most powerful group in southern Britain at the time of the Roman Conquest, they issued and used coins, and had many contacts with France.
They probably consisted of a group of tribes ruled by a single dynasty, their territory originally stretched from what is today West Sussex, Hampshire and Berkshire.
From about 15 BC, the Atrebates seem to have established friendly relations with Rome, and it was an appeal for help from the last Atrebatic king, Verica, which provided Claudius with the pretext for the invasion on Britain in AD 43. After the Roman Conquest, the territory of the Atrebates was divided up, with Silchester (Calleva Atrebatum) becoming the capital of a Roman civitas that administered the area of modern Berkshire, Oxfordshire, Surrey and north Hampshire.
The name Atrebates means 'settlers' or 'inhabitants'.
Reference: BBC History