Colchester (ancient, Camulodunum), county town and borough in Essex, eastern England, on the River Colne. The town of Colchester is the centre of trade for the agricultural areas of the borough, and it is also a small port. Industries include diesel and electrical engineering, printing, and insurance. The island of West Mersea, about 10 km (6 mi) south of the town, is renowned for its oysters. An 11th-century castle contains the largest Norman keep in England and houses an archaeological museum. Colchester is the seat of the University of Essex (1961).

Having flourished as the royal town of the British chieftain Cunobelinus (Cymbeline), Colchester was captured in AD 43 by Emperor Claudius I and became the first Roman colony in Britain. Much of the city wall built by the Romans still stands. In AD 60 and 61 the British queen Boudicca (Boadicea) rebelled against the Roman conquerors and destroyed the settlement. (Boudicca later committed suicide to avoid capture by the Romans.) Colchester soon revived to become one of the chief towns of Roman Britain. Later it was the Saxon stronghold of Colneceaster. During the early Middle Ages Colchester was a thriving river port. Flemish Protestants settled here in the 16th century and established textile industries. Population (1994 estimate) 149,621.