Star Magazine is blowing the lid off a secret Adam “DJ AM” Goldstein took to his grave — the celebrity spinner once enjoyed a fling with heiress Paris Hilton.

Adam’s former fiancee, socialite Nicole Richie, was beside herself with grief after learning of the DJ’s death of a rumored drug overdose in New York City last month. But that sorrow could soon turn to rage when Nic finds out that the man she still called a friend allegedly had sex with her lifelong best friend in September 2007.

According to a report in the Sept. 28 issue of Star, Paris seduced Nicole’s then-ex after running in to him at the Ultra Supper Club during the Toronto International Film Festival a couple of years ago. A frequent companion of Adam and Nicole’s during their two year romance, P.H. was in town filming her critically-panned camp musical Repo! The Genetic Opera — and she wasted no time “making eyes” at Nicole’s former lover and sobriety coach.

“She gave him a huge hug and kid, and he reciprocated,” says a spywitness, who recalls seeing Paris dancing seductively on Adam for much of the night. “They got more and more affectionate as the night went on, until they were all over each other. There was no doubt she wanted him.”

The two then allegedly retired to the DJ’s posh suite at the Metropolitan Hotel, where they did the mattress mambo until the break of dawn. A dishelved Adam emerged this following morning and promptly hailed a taxi to jet him to the airport.

Adam and Paris vowed never to tell Nicole — by then five months pregnant with her first child — about their one night stand. And for good reason. Paris, the heiress, and Nicole, the party-girl-turned-matured-mother-of-two, were inseparable as girls and grew up to claim reality show stardom as BFFs on the now-defunct reality series The Simple Life.

“Paris knows that the one thing you don’t do is sleep with your best friend’s ex…Paris crossed the line this time. This was the ultimate betrayal.”