Ricky Martin Files $20 Million Lawsuit Against Nephew Who Accused Him of Sexual Abuse

Ricky Martin says he's been "persecuted, besieged, harassed, stalked and extorted by a maladjusted person" -- AKA his nephew -- and he's now suing the guy who made wild accusations against him.

Martin filed a lawsuit Wednesday in San Juan against Dennis Yadiel Sanchez, who the singer claims is trying to "assassinate" his reputation.

Dennis had claimed back in July that he had a 7-month long romantic relationship with Ricky, and when it ended Ricky began stalking and harassing him -- something Ricky said was patently false. The nephew had filed a restraining order but when the heat was on him he withdrew the whole thing.

According to the new lawsuit, obtained by TMZ, the nephew hasn't stopped. Ricky claims a month after Dennis withdrew his restraining order, he began sending messages to Ricky through Instagram ... threatening to "assassinate his reputation and integrity" unless he anted up cash.

Ricky says in the lawsuit before Dennis filed for a restraining order, he was publicly boasting about being Ricky's nephew. Ricky says the nephew bombarded him with messages -- sometimes more than 10 in a single day -- for 4 months.

Ricky says the messages were clearly written by a "maladjusted individual."

Ricky says the nephew then went rogue, posting the singer's cell phone number on his Instagram. Ricky says the nephew also made an IG account for one of his children, which made him uncomfortable.

Ricky says he's lost numerous, multi-million dollar deals because of his nephew's false allegations, and now wants no less than $20 million to compensate the singer for his losses.