The black see-through dress with a blue trim was bought by an unidentified buyer Thursday in London. The auction price, which excludes a 20 percent buyers' premium, was originally expected to reach 8,000 pounds.
Some commentators maintain that the dress played a role in sparking the royal romance that started shortly after the event.
It was initially intended to be worn as a skirt, but Middleton wore it as a short dress with black lingerie underneath.
