Manhattan District Attorney Alvin Bragg's office said in court papers that the woman, who has not been identified publicly, came forward to prosecutors just days before the start of Weinstein's first trial but was not part of that case.
Prosecutors said they did not pursue the women's allegations after Weinstein was convicted and sentenced to 23 years in prison, but they revisited them and secured a new indictment after the state's Court of Appeals threw out his conviction last April.
Farber ruled in October to combine the new indictment and existing charges into one trial.
Weinstein's lawyers contend that prosecutors prejudiced him by waiting nearly five years to bring the additional charge, suggesting they had elected not to include the allegation in his first trial so they could use it later if his conviction were reversed.
Weinstein has denied that he raped or sexually assaulted anyone.