23 December 2015 Last updated at 15:06 GMT
The man climbed through a drive-through window on 19 December before threatening staff with the weapon.
The robber - whose face was covered by tights or stockings - left the Alfreton fast-food branch, in Derbyshire, with cash.
CCTV footage has shown the moment a man dressed as Santa Claus robbed a KFC restaurant at knifepoint.