The pleated dress is the same one that went billowing up over Monroe's legs in her famous scene standing atop a subway vent in the 1955 film "The Seven-Year Itch."
The dress was a part of a major collection of memorable Hollywood costumes belonging to actress Debbie Reynolds that was sold off Saturday night. CNN said Reynolds was in tears by the time the bidding culminated at $4.6 million plus a $1 million commission.
The identity of the buyer, who bid via telephone, was not revealed.
Three other Monroe costumes sold for another $2.7 million, CNN said.
Reynolds' collection also included costumes worn by Hollywood icons including Gary Cooper, Julie Andrews, Elizabeth Taylor and Charlie Chaplin.