"They're both thrilled," Johansson's rep Marcel Pariseau tells PEOPLE.
Reynolds, 31, currently filming the aptly named The Proposal in Boston with Sandra Bullock, recently popped the question to Johansson, 23. The couple have not set a wedding date.
The Nanny Diaries star is expected to show off her sparkler at Monday night's Metropolitan Museum Costume Institute Gala in New York. Sources say Johansson is eager to "show off her rock" with her Dolce & Gabbana gown.
Friends Give Their BlessingJohansson – whose debut album Anywhere I Lay My Head is due out May 20 – will next be seen in Woody Allen's Vicky Cristina Barcelona and the Drew Barrymore-produced He's Just Not That Into You. Reynolds, who was once engaged to singer Alanis Morissette, has upcoming projects including Fireflies in the Garden with Julia Roberts and X-Men Origins: Wolverine.
Mutual pal and Wolverine costar Hugh Jackman recently revealed he is protective of Johansson (they bonded working on 2006's Scoop and The Prestige) and gave Reynolds his blessing, calling him "one of the funniest guys I've ever met ... He's phenomenal."
Additional reporting by SAMANTHA MCINTYRE