The Royal Mail has revealed they will help him celebrate his big day by making special stamps with his princely face on.
Prince William will be the first member of the Royal Family to have stamps issued on their 21st birthday.
It's reckoned that loads of people all over the world will be desperate to get their hands on the 27p stamps, released just before the big day on 21 June 2003.
The Queen gets the final say on whether the stamps look right, with the Royal Mail and the Palace already thinking about the designs.
The Prince has been on stamps before, with the Queen, the Queen Mother and Prince Charles to celebrate the Queen Mother's 100th birthday in 2000.