Earlier this week Johansson was spotted wearing a diamond ring at the Venice International Film Festival in Italy.
She was previously spotted wearing an ornate gold ruby ring in July but friends dismissed reports she was engaged at the time, saying it was "simply a piece of jewellery".
Rumours first began to swirl that the couple were planning to wed after the blonde beauty was spotted wearing a huge pear-cut diamond ring as she wandered around New York with Romain, 30, in February.
The Avengers star admitted last year she liked being married but hinted that she wasn't interested in tying the knot again for quite a while.
She said: "I got married when I was young and it was incredibly romantic and I liked being married, actually.
"But it is different. It's hard to put into words. To me, being in a functioning relationship doesn't mean you have to be married.
"I never think about marriage. Is that weird? The only time I ever think about it is when people ask me. It's really not important to me."
- Bang! Showbiz