"They're engaged and very happy," a source says of the couple. A rep for the actress said, "I can confirm that Scarlett and Romain are engaged. They haven't chosen a date for the wedding."
The proposal occurred about a month ago. The ring is a vintage Art Deco style.
Earlier this week, Johansson, 28, was spotted wearing the diamond stunner while in Italy promoting her new sci-fi movie Under the Skin at the Venice International Film Festival.
Over the past year, Johansson had been spotted with diamond rings on her left hand. The last time was in July, when she arrived in Los Angeles wearing an ornate gold ring with a diamond and ruby. That bauble, though, was "simply a piece of jewelry."