The 26-year-old actress, who split from husband Ryan Reynolds last year, has been linked to Penn, 50, ever since they were spotted enjoying what appeared to be a lunch date back in February.
A representative for Johannson brushed off the speculation by insisting, "Scarlett and Sean are not dating - they have been friends for years," but the statement failed to silence gossip about the alleged romance.
Johansson and Penn have now fuelled rumours they are in fact a couple after they were photographed holding hands following the White House Correspondents' Association dinner in Washington, D.C.
The Lost In Translation star was wearing Penn's suit jacket around her shoulders as they walked hand-in-hand in the chilly night air.
