Monroe stood over a subway grate in the iconic dress.
It was sold as a part of a collection of Hollywood history, gathered by singer and actress Debbie Reynolds.
Also auctioned off was the red dress Monroe wore in "Gentlemen Prefer Blondes". It sold for $1.2 million.
Buyers also snapped up the cotton dress Judy Garland wore in "The Wizard of Oz", and Charlie Chaplin's bowler hat from "The Little Tramp."
More of Reynolds' treasure trove is set to be sold in December.