A source told Us Weekly that ScarJo, 26, "has essentially moved in over the last few weeks," though Johansson has denied pregnancy rumors.
Fifty-year-old Penn, meanwhile, has a daughter and son with ex-wife Robin Wright who are not much younger than Johansson.
But Johansson and Penn are far from the only May-December couple in Hollywood. Demi Moore and ex-husband Bruce Willis both married much younger spouses, and Hugh Hefner is set to marry 24-year-old Crystal Harris in June -- a woman who is SIXTY-ONE years his junior.
From Johansson and Penn to Hefner and Harris, let us know: Which May-December couple makes you say "ew"?