Sources directly connected with Justin Bieber tell TMZ ... the singer is adamant he never had sexual relations with the woman who claims she had his baby ... and as far as he knows, Justin has never even met her. 

The woman in question reportedly filed a paternity suit against Justin, claiming she and JB had a special hug backstage at a recent concert at the Staples Center in L.A. ... and 9 months later, she popped out a baby, baby, baby, oh. 

But we're told Justin believes the story was "completely made up."

BIeber just tweeted about the situation -- saying, "Im going to ignore the rumors...and focus on what is real. an opportunity to help by doing what i love. Judge me on the music! Love yall!"