The woman, who would only identify herself as Robin, was joined by attorney Gloria Allred at a news conference Tuesday ... becoming the third woman to come forward with similar allegations against the 83-year-old director.

Robin -- who also wouldn't say how she knew Polanski -- said she told a friend the next day about what happened and refused to tell her father because she was afraid he might do something that would send him to prison for the rest of his life. Allred said Robin did not go to law enforcement at the time but did so this year.

Robin said she's not over the sexual assault after all these years ... unlike Polanski's first victim, Samantha Geimer, who supports Polanski's full freedom.

Allred said they will not file a civil lawsuit against Polanski and will explore their next step.

As we've reported ... Polanski had sex with Geimer when she was 13 in 1977 ... but she's since said what's happening to Polanski is outrageous because if she's over it ... everyone else should be, too.

Charlotte Lewis came forward in 2010 alleging Polanski forced himself on her in 1982 when she was 16. Allred repped her at the time but Polanski wasn't charged. Allred declined to answer whether she still reps Lewis.