The actress wore the red polka-dotted dress to present the 'Best Actor' Oscar, which went to The Artist's Jean Dujardin.
According to the New York Post, an unknown British couture collector in London paid for the size four Dior dress on 1stdibs.com.
The Black Swan star apparently borrowed the dress from couture dealer Rare Vintage, who sold the dress on 1stdibs.com post-Oscars night.
Portman, who previously suggested she may retire from acting to raise her family, is set to return to work after joining two Terrence Malick films.
"Audrey [Hepburn] made some good choices in life... More and more I realise how unimportant it is to be in the history books or anything like that," she explained. "Putting time and energy into your children - that's valuable."
Watch the trailer for Black Swan below: