John Travolta Triumphs Against Author of 'Bathhouse Sex' Book

John Travolta just won a big legal victory against the author of a book who claims the actor frequented gay bathhouses ... TMZ has learned.

You'll recall, Travolta was sued for libel by Robert Randolph -- the author of "You'll Never Spa in This Town Again" -- who claimed Travolta and his lawyer conspired to discredit him and his book by spreading lies about his mental health.

Randolph's beef stems from a letter fired off and allegedly leaked by Travolta's lawyer Marty Singer, which stated Randolph suffers from mental problems -- and claimed his book was packed with lies.

The judge dismissed the case today -- agreeing with Travolta and Singer's lawyer Lynda Goldman -- ruling the letter was a legitimate part of the legal dispute over the book, and therefore protected by the First Amendment.