The House of Percy (old French Perci) is an English noble family. They were the most powerful noble family in northern England for much of the Middle Ages, known for their long rivalry with another powerful northern English family, the House of Neville.
The House of Percy descended from William de Percy (d. 1096), a Norman who crossed over to England after William the Conqueror in early December 1067, was created 1st feudal baron of Topcliffe in Yorkshire, and was rebuilding York Castle in 1070. The name derives from the manor of Percy-en-Auge in Normandy, the home of the family at the time of the Norman Conquest. Members have held the titles of Earl of Northumberland or Duke of Northumberland to this day, in addition to Baron Percy and other titles. The Percy surname twice died out in the male line but was re-adopted by the husband of a Percy heiress and by their descendants. In the 12th century, the original Percy line was represented by Agnes de Percy, whose son by her husband Joscelin of Louvain adopted the surname Percy. Again in the 18th century, the heiress Elizabeth Seymour married Sir Hugh Smithson, who adopted the surname Percy and was created Duke of Northumberland.