Police are hunting for vandals who painted a face mask on the ancient chalk figure known as the Long Man of Wilmington.
The unauthorised addition to the 235ft landmark carved into the hillside between Eastbourne and Lewes in East Sussex was first spotted on Tuesday.
Dog walker Jeremy Christey tweeted: "The Long Man of Wilmington, Sussex has inherited a rather slender mask. Good man.
"To be safe, he probably needs to pull it down a touch..."
Sussex Police said that the mask painting was "unacceptable" even if it was intended as a humorous act.
“The Long Man of Wilmington is protected by law as a Scheduled Ancient Monument for its historical significance,” said Sergeant Tom Carter of the rural crime team.
"On top of this the figure is well known and enjoyed by the local community and this criminal damage is an affront to those who work to maintain this heritage asset for the enjoyment of all."
The Long Man was restored to its maskless original state by Saturday morning.
Its origins remain unclear although it dates back to at least 1710 and remains Europe's largest portrayal of the human form.
Mark Harrison, of Historic England, said: "Historic England is working closely with Sussex Police Rural Crime Team to identify the offenders who have caused damage to this protected archaeological site.
"We will also be liaising with the owners in order to provide advice and guidance to restore the Long Man of Wilmington."
Additional reporting by Press Association
