The see-through dress worn by Kate Middleton at a charity fashion show at St Andrews University has been sold for £65,000 plus £13,000 buyer’s fees.
London auctioneer Kerry Taylor had estimated the knitted dress would fetch between £8,000 and £10,000.
The prince was in the audience when Miss Middleton hit the catwalk wearing the dress with black lingerie at the university in Fife in 2002.
The pair, who met at St Andrews, will marry at Westminster Abbey next month.
The garment cost the mystery buyer a total of £78,000.
A man at the auction bought the dress on behalf of an individual he would only name as “Nick from Jersey”.
He said of the buyer: “He thinks it’s an iconic piece and is very happy with the purchase.”
Ms Taylor had started the auction at £20,000, with plenty of interest via phone bids, but when the price reached £50,000 it became a two-way battle between the man in the room and some pre-registered bidders.
The dress was sold by Charlotte Todd, who created it while studying fashion design at the University of the West of England in 2000.
She initially intended it to be a skirt, but the show’s organisers decided it would be worn as a dress.
Mrs Todd, 31, who works in Bristol as an aquarium retail manager, said after the sale: “I’m completely shocked, I need to sit down and get my head round it.
“I just wanted to get out and get some fresh air during the sale. I wasn’t expecting it. I was turning round to see who was at the back of the room and what was happening.
“But I didn’t like to get my hopes up, I was thinking it might not sell. I wasn’t thinking of a sum of money in my head.”
She said she and her husband would probably put the money towards buying a house before joking: “Maybe I should rustle up some more dresses?”
The garment had been stored in a box at the bottom of her mother’s wardrobe for eight years.
Speaking before the sale, auctioneer Kerry Taylor said the huge interest in the dress reflected the “Kate-mania” sweeping the world.
“I am quite astonished by the amount of interest from all over the world, from Korea to Japan to America,” she said.
“Literally there’s global interest in the dress and that reflects the interest in Kate Middleton.
“I feel quite sorry for her – if you’re getting this kind of response to a dress that she wore for 10 minutes all those years ago it’s quite extraordinary.”
The Passion For Fashion auction, held in central London on Thursday, featured more than 250 lots, including two gowns worn by Prince William’s mother, Princess Diana.
A Catherine Walker white lace evening gown, worn by the princess during a visit to France in 1988, made £36,000, including the buyer’s premium.