William, who is second in line to the throne, gains access to the interest and other income accrued on the $13 million he was left by his mother, who died in a Paris car crash in 1997.
William's Clarence House office would not say what the prince planned to do with the money, estimated at $500,000 to $600,000 a year.
Harry, 22, William's brother, will gain access to a similar amount when he turns 25. According to Diana's will, the brothers gain access to the capital sum when they turn 30.
William and Harry are both second lieutenants in the British army, earning salaries of about $44,000 a year. They also receive money for accommodation and other expenses from their father, Prince Charles.
Now that he is 25, William also can marry without the consent of his grandmother, Queen Elizabeth II — but no royal nuptials are on the horizon. William and his long-term girlfriend, Kate Middleton, announced in April that they had split.