Brett Ratner is suing a woman who claims the director raped her in L.A. over a decade ago ... calling the allegations a total lie.

According to the suit ... Melanie Kohler claimed in an October 20 Facebook post, "Brett Ratner raped me" and continued by calling him "a rapist on at least one night in Hollywood about 12 years ago."

She claimed he "preyed on me as a drunk girl [and] forced himself upon me" ... according to the docs.

Ratner's lawyer tells us Kohler said she had met the director at an L.A. club in either 2004 or 2005, and he took her to Robert Evans' home where Ratner was living at the time. She says he forced her to have sex.

Ratner's lawyer threatened to sue if if she didn't take the post down immediately, and we're told she did, but recently started putting the same story out in various ways.

Six women, including actresses Natasha Henstridge and Olivia Munn, claim Ratner sexually harassed or assaulted them.