Jessica Mann detailed a catalogue of abuse by the Hollywood producer, saying he once trapped her in a hotel bedroom and raped her.
Three of the five charges against Mr Weinstein relate to Ms Mann.
He denies non-consensual sex and his lawyers say emails prove his and Ms Mann’s relationship was consensual.
Ms Mann’s evidence came at the end of the fourth week of the Manhattan trial of the Oscar-winning Hollywood mogul, who produced films including Shakespeare in Love and The English Patient.
The 34-year-old said she met him in late 2012 or early 2013 at a party, and she told him of her ambition to be an actress. Later, she said, he invited her and a friend to a Los Angeles hotel suite. He then allegedly pulled Ms Mann into a bedroom and performed oral sex on her.
Ms Mann then began a relationship with Mr Weinstein. “I entered into what I thought was going to be a real relationship with him and it was extremely degrading from that point on,” she said.
She said he once urinated on her, and in 2013 raped her in a Manhattan hotel room. “If he heard the word ‘no,’ it was like a trigger for him,” she said.
When asked why she stayed in the relationship, Ms Mann said in tears that there was “no short answer”.
“One of the aspects initially was that I had had a sexual encounter” with him, she said. “That wasn’t something I could undo. That really confused me and hurt me.” She stayed with him partly out of fear, she said.