The most iconic item -- the strapless dark blue velvet gown worn at a 1985 dinner thrown by US president Ronald Reagan in honour of the Prince and Princess of Wales -- raised £240,000 (281,000 euros, $363.000).
It was immortalised in the photographs of Diana dancing with Travolta to the song "You Should be Dancing" from his film "Saturday Night Fever".
The dress was bought "by a British gentleman who said he wanted to buy it as a surprise to cheer up his wife", explained auctioneer Kerry Taylor.
The garments, created by some of Diana's favourite designers including Zandra Rhodes, Catherine Walker, Bruce Oldfield and Victor Edelstein, share an extraordinary history.
Some were worn by the princess during official trips to Austria, Australia, Brazil, India, South Korea and the United States, said the auction house.
They were then acquired by Florida businesswoman Maureen Rorech at a 1997 sale to raise money for humanitarian charities supported by the princess, two months before she was killed in a Paris car crash.
Rorech put 14 of the dresses up for sale at a Canada auction in 2011 after declaring bankruptcy, but only four sold as the "reserve prices were ridiculously high", said a spokesman for Kerry Taylor.
All of the remaining items sold on Tuesday at prices ranging from £24,000 to £240,000. Two were bought by an "important" British museum, revealed Taylor.
"It's important for the generations to come," she added. "Diana was the people's princess, so the people should be able to see these dresses. This is our heritage, our history."