Mad Madge! Madonna Blindsided By Sean Penn & Naomi Campbell's SECRET Trysts

Sean Penn and Madonna recently rekindled their once-torrid love affair, but RadarOnline.com has learned that a new cheating scandal is already tearing the two apart.

RadarOnline.com can reveal that Penn is chasing around another secret lover from his playboy past: supermodel Naomi Campbell!

"He has been sleeping with Naomi behind her back," a source told RadarOnline.com. "They were together in Paris in early December and were hooking up then. They both have done their best to be discreet as Sean knows Madonna would be p**sed if she found out."

Penn and Campbell have a very long history together. In 1994, the actor reportedly cheated on his then-wife Robin Wright with the English model.

Now, RadarOnline.com can reveal the two hooked up at a secret meeting in New York on Sept. 28, when Penn was in town for the Clinton Global Initiative and Campbell appeared at Fashion 4 Development's "First Ladies Luncheon."

And on Dec. 5, Penn and Campbell rendezvoused at a private dinner he held on Dec. 5 at the luxe eatery Les Bains Douches in Paris.

Madonna, 57, and Penn, 55, had a famously chaotic marriage that ended in divorce in 1989, and it remains to be seen how the pair's relationship will play out amid these latest revelations.

Campbell's reps denied that the supermodel is carrying on a fling with Penn. But, perhaps in a sneak peek of things to come, Madonna called Penn and her other estranged ex-husband, Guy Ritchie, "a**holes" during her Rebel Heart Tour concert in Miami on Jan. 23.