The ivory pleated dress which blew up shockingly revealing Monroe's pants in Billy Wilder's 1955 classic was sold to an anonymous bidder for £2.65 million ($4.3 million).
The Marilyn Monroe classic was the premier lot at the auction of actress Debbie Reynolds' film memorabilia in Los Angeles.
Monroe's red sequined dress and feathered headdress from 1953's Gentleman Prefer Blondes also went for £803,000 ($1.3 million) in the sale.
A pair of red slippers designed for JUDY GARLAND but never actually worn by the star in the Wizard of Oz sold for £989,000 ($1.6 million) and Charlie Chaplain's bowler hat from The Little Tramp fetched £84,000 ($135,300).
The male Phil Campbells, joined by one woman called Phyllis Campbell, had initially planned to travel to their tiny namesake Alabama town this weekend to help celebrate its 100th birthday.
But a fearsome tornado struck Phil Campbell, the town, on April 27, and the Phil Campbell contingent decided to pitch in and help out.
One Phil Campbell, a pastor from Coffs Harbour, Australia, said: “When the tornado hit, this trip went from being whimsical to necessary.”
US President Barack Obama visited the state after the deadly tornadoes struck.
On Saturday, Phil Campbells from across the United States, Australia and Britain received celebrity treatment at their third-ever gathering in the town.
Wearing "I'm with Phil" T-shirts, they rode in the anniversary parade, ate barbecue at the First Baptist Church and were honoured on stage at the community event.
But thoughts of the storm that killed 28 of the town's residents were never far away.
Fred Dummar, who lost his home, his restaurant and seven neighbours, wiped away tears during the programme.
"My friend died," said 8-year-old Makayla Baker, as she handed a baseball to Phil Campbell of Glasgow, for him to sign.
The effort to unite the Phil Campbells of the world began with a man by that name in Brooklyn, New York.
The 38-year-old first heard about the Alabama town - named for the railway engineer who placed a depot there - on a television show.
"I became obsessed with the town and wanted to gather all of the world's Phil Campbells together," he said.
A group of them first came together in 1995, and again in 1996.
The April tornado turned the reunion into a relief effort and so far the Phils and Phyllises have raised almost £25,000 ($40,000).
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