The images, apparently taken by the actress, follow the leaking of naked photos of another star, Jessica Alba.
The FBI said it was investigating "the person or groups responsible for a series of computer intrusions involving high-profile figures".
Hacked photos of Justin Timberlake and his friend Mila Kunis have also been found online, according to reports.
Laura Eimiller, of the FBI's Los Angeles office, said dozens of stars had been the victims of hackers who taken not just photos but personal details of many celebrities.
She could not name anybody "due to the pending nature of our investigation".
One leaked photo appears to show Johansson topless on a bed while another apparently shows her wearing a towel while revealing her bottom.
Where do we put the line under this problem of hacking? Should a more serious punishment be brought in to stop these people invading private spaces?
