Diddy is facing a new rape allegation, one that dates back to before he was famous … and the alleged victim says it went down during an already infamous NYC event.

The accuser, who’s filing as a Jane Doe, says she was attending a December 1991 charity basketball event at the City College of New York, which was co-sponsored by a then up-and-coming producer, Sean “Puffy” Combs.

She and a friend were invited by an unnamed rapper set to appear at the event, she claims in the lawsuit — when she and her friend arrived, she says the crowd outside was large and getting unruly, and she convinced a security guard to let her and the friend in early.

Inside, they couldn’t find the rapper, and the woman claims they were led to an office of the gym locker rooms, where Combs had a makeshift dressing room. She says she didn’t know who Puffy was at the time, and asked if he could help her meet up with her rapper friend — and Diddy said he’d make it happen.

However, she alleges Combs handed her a plastic cup he told her contained Coca-Cola, and she took a sip, immediately started feeling woozy and got up to leave — but she claims Combs blocked her.

According to the suit, filed by her attorney Tony Buzbee, he started touching himself and fondling her … and overpowered her when she started struggling. She alleges he pushed her down, tore off her underwear and then raped her.

Afterward, she says she told Combs she was going to tell her rapper friend what had happened — and she claims Combs threatened her not to do that, because “people can come up missing.”

The accuser says Combs then left to deal with something at the event … and she found her friend and they made their exit. In the suit, she says the scene outside was chaotic, as people were running in every direction.

She’s referring to a stampede that broke out because the ’91 event was oversold — 9 people ended up dying, and many more were injured. The headliner that day was the late great Heavy D, who was signed to Uptown Records … the label where Combs got his start.