Graffiti was sprayed over the ruins of the C12th castle last week.
And Wakefield Council, which manages the monument, will now have to find the funds to have it cleaned.
Coun Jacquie Speight, Wakefield Council's cabinet member for culture, leisure and sport, said: “It is utterly disgusting that graffiti has been sprayed on the monument.
"It is impossible to understand why anyone would wish to do this at Sandal Castle, which is an important part of our heritage.
“We are working with Historic England to identify the specialist contractors needed to deal with and remove the graffiti.
"We are doing this as quickly as possible but it is a specialist service and it won’t be cheap or easy to do.
“I would urge anyone who sees vandalism taking place to report it immediately 0345 8 506 506, and help us stop this mindless behaviour.”
Vandals also defaced the site last September.
They sprayed graffiti over the medieval castle walls and, days later, recklessly drove a car on to its hillside.
The council announced new measures to combat the anti-social behaviour and urged people to help protect and respect the site.
At the time, the authority said it would install temporary CCTV and increase patrols.
Police said they believe the latest incident occurred on July 19.
Anyone with information should is asked to call 101, quoting crime reference 13170334162.