Gwyneth Paltrow claims that Brad Pitt threw Harvey Weinstein against a wall and told him "I'll kill you" after the producer made a pass at her.
She claims the producer placed his hands on her and suggested they head to a bedroom for massages.
According to the actress, Mr Pitt threw Mr Weinstein against a wall at the opening night of Hamlet on Broadway in 1995.
Ms Paltrow made the remarks on Wednesday while appearing on the Howard Stern Show and said the incident occurred just after she had signed onto work on Emma and Shakespeare in Love.
She said that Mr Pitt "leveraged his fame and power" to protect her at a time she "didn't have fame or power."
Ms Paltrow added that "Harvey was never inappropriate again" following the incident.
Mr Weinstein has "unequivocally denied" all allegations of non-consensual sex.
Ms Paltrow previously told the New York Times that Mr Pitt had words with the producer over his behaviour.
She told the newspaper that she had received a fax telling her to go to his hotel room when he then made a pass at her.
After she rejected his advances, Mr Weinstein told her never to tell anyone what happened between them.
Ms Paltrow told Howard Stern "He screamed at me for a long time. It was brutal."
Some 80 women, including prominent actresses, have accused Mr Weinstein of miscoduct ranging from rape to harassment.