Jake Paul is not only DENYING he sexually assaulted TikTok star Justine Paradise -- but he's vowing to sue her defamation.

24-year-old Paradise posted a video on YouTube last week alleging Paul forced her to perform oral sex on him after she made it clear she did NOT consent.

In her video, Paradise says she was hanging out with Paul at his L.A.-area home when they began to kiss -- something she DID consent to doing.

But, Paradise claims Paul pushed for sexual intercourse and she shut him down ... so he continued to aggressively push for oral sex.

Paradise claims she again declined -- telling him "no" multiple times -- but he forced himself on her anyway.

In her video, Paradise never mentions the specific date of the alleged incident -- but notes they began to hang out in June 2019, implying the incident took place sometime after that.

Now, Paul is firing back in a statement through his attorney Daniel E. Gardenswartz -- calling BS on the whole story and vowing to take legal action of his own.

"While others have already begun to debunk the claim alleged against him, our client categorically denies the allegation and has every intention of aggressively disproving it and pursuing legal action against those responsible for the defamation of his character," Gardenswartz said.

"Our client believes that any false allegations diminish the credibility of those who have truly been victims of misconduct."

In the meantime, Paul is training for his upcoming boxing match against UFC star Ben Askren -- which is expected to go on as planned despite the looming allegations.