town (parish), Breckland district, administrative and historic county of Norfolk, England. The town lies on the edge of Thetford Chase Forest. It possesses the remains of a Cluniac priory, a Benedictine nunnery, and a large medieval mound known as Castle Hill. Excavations have revealed traces of an Anglo-Saxon town of about 4,000 inhabitants with considerable industry and a mint.

Officially designated an “expanded town” for London's overspill population, Thetford has grown since 1960. It has a number of industries, including paper and pulp manufacture, canning, and light engineering. Pop. (2001) 21,760.