The costume from The Seven Year Itch was purchased by an as-yet-unidentified phone bidder in an auction in Los Angeles over the weekend.
Synonymous with Monroe for many fans, the dress has set sail a million fancy dress costumes and copycat film moments.
It was the highest-grossing lot in an auction of film memorabilia owned by actress Debbie Reynolds. Auctioneers expected the dress, which Reynolds purchased for $200 in 1974, to sell for no more than $2m.
The auction disposed of other memorable film costumes that Reynolds had accumulated since 1970. A red sequinned dress and feathered headdress that Monroe wore in Gentlemen Prefer Blondes sold for $1.47m, and bidding for Audrey Hepburns Ascot dress and hat from My Fair Lady reached $3.7m. Elizabeth Taylors Cleopatra headdress, Charlie Chaplins bowler hat and Judy Garlands blue gingham dress from The Wizard of Oz also found buyers.
Reynolds decided to auction her extensive collection of film memorabilia in light of mounting maintenance costs. The actress, who starred in Singin in the Rain, had hoped that her collection might form the basis of a permanent collection for a Hollywood museum. Reports stated that she was in tears when the auction on Monroes white dress closed.More