The 33-year-old slice of wedding cake from the marriage of the Prince of Wales to Lady Diana Spencer in 1981, has been sold for $1375 (£828), reported the Telegraph.
According to the report, the cake, still in its original white and silver presentation box, was sold online by Nate D Sanders Auctions of Los Angeles.
The box was decorated with Prince of Wales feathers and monogrammed with "CD".
It came with a card, saying: "With best wishes from Their Royal Highnesses, the Prince & Princess of Wales."
Sam Heller, spokesman for the auction house, said the buyer was a private collector.
Although the cake came wrapped in its original wax paper, Mr Heller said it wouldn't be a good idea to try to eat it.
He added, however, that there was a small but dedicated group of royal cake collectors.
Some, Heller said, had bought purchased cakes dating to the days of Britain's Queen Victoria, who married in 1840.
A slice commemorating the marriage of Princess Diana and Prince Charles sold for £1000 six years ago, even though it came not from the five-tier cake served to wedding guests but from one of 22 others distributed among royal staff.