A third woman has come forward to accuse the film director Roman Polanski of sexual abuse when she was a minor.

The woman, who was identified only as Robin, held a news conference in Los Angeles on Tuesday with the attorney Gloria Allred. She said that Mr. Polanski “sexually victimized” her in 1973, when she was 16. She is now 59.

In 1977, Mr. Polanski pleaded guilty to unlawful sex with Samantha Geimer when she was 13. He fled the country before his sentencing in 1978. His lawyers have been fighting to lift an international arrest warrant, and he has not returned to the United States since it has been in effect. In June, Ms. Geimer asked Judge Scott M. Gordon of Los Angeles Superior Court to drop the case against Mr. Polanski, 83. That case is still pending.

At the news conference, Robin said the possibility that the case might be dropped “infuriated” her.

“I am speaking out now so that Samantha and the world will know that she is not the only minor Roman Polanski victimized,” Robin said, reading from a prepared statement. She said that the day after it happened, she told one friend, but otherwise has kept it to herself for 44 years.