Johansson's rep, Marcel Pariseau, confirmed her engagement to Romain Dauriac on Thursday. Pariseau said no date was set for their wedding.
Johansson, 28, whose marriage to actor Ryan Reynolds ended in 2010, is generally reluctant to talk about her private life, but has explained what she does and doesn't want in a relationship.
"I don't like jealous behavior," she told Marie Claire magazine in an interview earlier this year. "It's really unattractive because it shows a sort of insecurity."
Johansson starred in "Lost in Translation" and was in Italy this week promoting "Under the Skin" when the large diamond ring on her left hand started getting attention.
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