A clerk for Granite County confirmed to People Magazine that the couple secretly exchanged vows in Philipsburg, Montana on 1 October.
It is thought the ceremony took place a month after Scarlett gave birth to the couple's daughter, Rose, and a year after they got engaged.
The 30-year-old was previously married to Hollywood actor Ryan Reynolds.
Rumours that Scarlett and Romain had tied the knot had been circulating for a while, after she was photographed wearing a ring on her wedding finger.
The star of Lost in Translation and The Avengers was left "devastated" following her divorce from Ryan Reynolds in December 2010.
Speaking to Vogue magazine about the end of her two-year marriage, she said: "It really throws you. You think that your life is going to be one way, and then, for various reasons or whatever, it doesn't work out."
Newsbeat has contacted Scarlett's agent, but has not received a response so far.
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Scarlett Johansson has reportedly married her fiancé, French journalist Romain Dauriac.