Nick Carter Claims New Woman Suing for Alleged Sexual Assault On Yacht Is Lying

Nick Carter is pushing back against a woman who claims he sexually assaulted her on his yacht when she was 15 years old ... he says her lawsuit is full of false allegations.

The Backstreet Boys singer is being sued by a woman who says Nick sexually assaulted her multiple times back in 2003, when she was 15 and he was 23 -- she claims Nick forced himself on her aboard a yacht, and on a bus after plying her with alcohol.

But Nick is vehemently denying it happened, claiming the lawsuit is full of BS.

This accuser has filed her lawsuit anonymously, and she's now the third woman suing Carter for sexual assault.

Carter's lawyer, Dale Hayes Jr., tells TMZ ... when the woman made her allegations against Nick 2 decades ago, authorities investigated her claims and found no merit to move forward with charges.

Nick's lawyer says the accuser is simply "repeating the same false allegations in a new legal complaint" but he says that "doesn’t make them any more true."

As we first reported, Nick is also being sued by a woman named Shannon Ruth, who filed a lawsuit in December claiming he sexually assaulted her after a concert in 2001, when she was underage ... but he says he has witnesses who will testify Ruth is lying.

Singer Melissa Schuman is also suing Nick over an alleged 2002 sexual assault. He denies her allegations, as well.