Justin Bieber's Alleged Baby Mama Mariah Yeater -- The Mug Shot

TMZ has obtained the mug shot for the woman who claims she's Justin Bieber's baby mama ... taken moments after she was arrested for allegedly slapping her ex-BF last year ... the OTHER guy she accused of fathering her child.

As we previously reported, Mariah Yeater was charged with battery in December 2010 for slapping her ex-BF John Terranova ... after she told the guy he was the father of her then-unborn baby.

John claims the baby isn't his -- ditto for Bieber.