According to new legal docs, obtained by TMZ, the Backstreet Boys member says he has 12 people who have testified the alleged victim's version of events can't possibly be true.

TMZ broke the story ... Nick is being sued by a woman named Shannon Ruth, who filed a lawsuit in December claiming he sexually assaulted her after a concert way back in 2001 when she was underage.

But, Nick says not so fast ... he claims he has 12 witnesses from the concert -- fans, security personnel and a talent manager -- who say the accuser's story is "factually impossible."

Remember ... Ruth, who says she is autistic and has cerebral palsy, claims she got in line to get Nick's autograph after the show, when he allegedly invited her back to his tour bus, plied her with alcohol and forced her to perform oral sex on him.

But, Nick says his dozen witnesses testified it was impossible for him to interact with the alleged victim after the concert ... they say there was no autograph line and fans had no access to the tour bus staging area.

What's more, Nick says one of Ruth's close friends at the time testified Ruth is lying about being autistic and suffering from cerebral palsy ... and she doesn't even believe Ruth was at the concert.

In the docs, Nick claims another witness testified they were on calls where Ruth's name was brought up as part of a plot to extort Nick ... an alleged money grab he's previously mentioned in his countersuit against Ruth.