People.com said wedding-bell buzz began after the "Hitchcock" and "Cat on a Hot Tin Roof" star was seen wearing a pear-shaped diamond ring in New York Monday.
"Scarlett is absolutely not engaged," her publicist told People.com Wednesday.
Johansson was married to actor Ryan Reynolds for two years before they divorced in 2010.
Reynolds has since married actress Blake Lively.
Justin Bieber crashes Drake Bell's album release party
Members of Congress to keep receiving porn magazine