According to new legal docs, obtained by TMZ, a judge in New York just ruled a woman suing Diddy for an alleged sexual assault she claims happened 20 years ago must reveal her identity or have her suit dismissed.

The woman is suing as a Jane Doe ... and her lawsuit is one of the 120 suits Texas attorney Tony Buzbee says he's filing against Diddy.

In the ruling, Judge Mary Kay Vyskocil says ... although the allegations are sensitive in nature, the Jane Doe has not shown she is entitled to remain anonymous ... pointing out the woman is an adult who "decided to file a lawsuit in which she accuses a famous person of engaging in heinous conduct approximately 20 years ago."

The judge says the woman must file a complaint in her own name by November 13 or the case will be thrown out of court.

As we first told you ... this accuser was the first Buzbee client to file suit against Diddy, claiming he raped her under threat of violence in 2004 when she was a college freshman.

For his part, Diddy denied the allegations ... saying he's never sexually assaulted anyone.