Thetford Castle is a medieval motte and bailey castle in the market town of Thetford in the Breckland area of Norfolk, England. The first castle in Thetford, a probable 11th-century Norman ringwork called Red Castle, was replaced in the 12th century by a much larger motte and bailey castle on the other side of the town. This new castle was largely destroyed in 1173 by Henry II, although the huge motte, the second largest man-made mound in England, remained intact. The motte, recognised as a scheduled monument, now forms part of a local park, and the remains are known variously as Castle Hill, Castle Mound and Military Parade.

A motte-and-bailey castle is a European fortification with a wooden or stone keep situated on a raised area of ground called a motte, accompanied by a walled courtyard, or bailey, surrounded by a protective ditch and palisade. Relatively easy to build with unskilled labour, but still militarily formidable, these castles were built across northern Europe from the 10th century onwards, spreading from Normandy and Anjou in France, into the Holy Roman Empire, as well as the Low Countries it controlled, in the 11th century, when these castles were popularized in the area that became the Netherlands. The Normans introduced the design into England and Wales. Motte-and-bailey castles were adopted in Scotland, Ireland, and Denmark in the 12th and 13th centuries. By the end of the 13th century, the design was largely superseded by alternative forms of fortification, but the earthworks remain a prominent feature in many countries.