8:04 AM PT -- Nelly's attorney, Scott Rosenblum, tells TMZ ... "It comes as no surprise that Ms. Green filed a lawsuit against Nelly seeking money after we announced our intention to hold her accountable. We always believed her accusation was motivated by greed." He adds, Nelly plans to countersue.

Nelly's rape accuser, who refused to testify in the criminal case against him, has now filed a lawsuit against the rapper ... claiming sexual assault and defamation.

Monique Greene says she had worked as a host at the Seattle club where Nelly performed on Oct. 6, 2017 ... although she was not on the clock that night. She says after the show she was drinking alcohol, and hanging out with Nelly and his entourage. According to the docs, obtained by TMZ, Nelly invited her to join him at an after-party, and she got into an SUV with him.

She says the SUV brought them to Nelly's tour bus which was parked in a Walmart parking lot. She boarded and walked to the back of the bus ... where Nelly's bedroom was. Greene says she sat down on the bed, and Nelly started masturbating.

According to the suit, Nelly then had sex with Greene against her will ... both orally and vaginally. Greene says she tried to dissuade him by saying he needed to wear a condom, but he refused and continued assaulting her. Afterward, Greene says she was screaming she wanted off the bus ... and an entourage member pushed her off, and Nelly threw a $100 bill at her and said, "Bye bye."

She says Nelly taunted her from the bus as she stood in the parking lot. Greene says she called an Uber, and while waiting she also called 911. Police showed up, interviewed Greene and Nelly ... and arrested him for 2nd degree rape. Greene says police took her to a medical center for treatment and to complete a rape kit.

Greene is also suing for defamation, claiming after his arrest Nelly and his team began a smear campaign ... saying she was only after "money, fame and notoriety."

As we first reported ... prosecutors dropped the criminal case against Nelly when Greene refused to testify. In her lawsuit, she says she felt she couldn't "stand up against a celebrity and that the criminal system would fail her."