A judge ruled Friday that one additional woman who says Bill Cosby sexually assaulted her will be permitted to testify at his trial later this year on charges that he sexually assaulted a former Temple University staff member in 2004.

Legal experts have said that the account of a second woman describing what she said was an act of sexual assault could strengthen the case against Mr. Cosby, 79, who has denied any misconduct.

But the judge’s decision was far from a full victory for prosecutors in Montgomery County, Pa., who had sought to introduce testimony from 13 women who have accused Mr. Cosby of assaulting them. The prosecutors had told Judge Steven T. O’Neill of the Court of Common Pleas that the testimony of the group of women was important in showing a pattern of conduct by Mr. Cosby that bolstered the account of Andrea Constand, the former Temple employee.

Judge O’Neill agreed to allow the testimony of the one woman, identified only as “Prior Alleged Victim Six,” in a one-page decision that briefly discussed his legal reasoning. The judge said that he had considered the evidence, the legal arguments and aimed for “a careful balancing of the probative value of the other acts evidence and the prejudice to the Defendant.”