German media outlet Der Spiegel reported on Friday that American woman Kathryn Mayorga had claimed Ronaldo sexually assaulted her in a hotel room nine years ago.
Der Spiegel said Ms Mayorga, 34, filed a report with Las Vegas police shortly after the alleged incident.
Juventus forward Ronaldo has strongly denied all of the claims and appeared in an Instagram video over the weekend, where he said: "No, no, no, no, no".
Earlier on Wednesday, Ronaldo posted on his Twitter account: "I firmly deny the accusations being issued against me".
Ronaldo on his own part has vehemently denied the allegations which emerged in German magazine Der Spiegel. "Rape is an abominable crime that goes against everything that I am and believe in", he wrote in a tweet.
Real sold Ronaldo to Juventus for £99 million in July and while it took the 33-year-old a little while to get going in Italy, nobody has had a greater contribution than him in Serie A this term.
"The psychological trauma of the sexual assault, the fear of public humiliation and retaliation and the reiteration of those fears by law enforcement and medical providers left plaintiff terrified and unable to act or advocate for herself", the lawsuit claims. 'You want to promote by my name.
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She was invited to join guests in a hot tub but she declined because she did not have a bathing suit, the complaint said.
Her lawyer is holding a media conference later on Wednesday in Las Vegas. "At this time she's emotionally fragile".
"She came to us and we had concerns with what happened in 2009 ... with the police investigation and the circumstances around the settlement and how the negotiations were conducted", Drohobyczer told AP.
According to court papers, Ronaldo apologized after the alleged attack, telling Mayorga, "Sorry, I'm usually a gentleman".
Meanwhile, the LVMPD says they've reopened the investigation - but Ronaldo has not been charged with a crime.
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The lawsuit seeks unspecified damages of at least $200,000 US.
Ronaldo in action for new club Juventus.