Cristiano Ronaldo is striking back against sexual assault allegations… after a Nevada woman sued the global soccer star for an alleged rape at a Las Vegas hotel in 2009.

Ronaldo - considered one of the world's greatest soccer players - Tweeting Wednesday: "I firmly deny the accusations being issued against me.

Rape is an abominable crime that goes against everything that I am and believe in." Kathryn Mayorga sued the Juventus player last week-- seeking more than $200,000 in damages.

AND seeking to void a $375,000 settlement that she says she was coerced into signing to keep quiet.

The lawsuit alleges that Ronaldo - a day after meeting Mayorga at a Las Vegas club - invited her, and others, to the penthouse suite where he was staying.

According to the lawsuit, Ronaldo asked the woman to perform a sexual act on him in a bathroom as she was changing - then allegedly pulled her into a bedroom and raped her.

On Friday, Ronaldo's lawyers threatened to sue a German magazine that published the accusation.

The Las Vegas Police Department says it's reopened an investigation into the alleged 2009 sexual assault.... but declined to say whether Ronaldo was the subject.