Travis Barker -- Hittin' Skins ... With UFC's Arianny Celeste

Travis Barker has decided to enter the UFC ... sorta ... 'cause he's dating the league's most famous ring girl, Arianny Celeste.

Sources close to the couple tell us ... the two have been dating for several months. We're told they met at one of Travis' concerts and the sparks flew from there.

Ever since, we're told they've been "inseparable" ... and Travis is telling friends he thinks she could be "the one."

In fact, Travis recently introduced Arianny to his kids -- which is a pretty HUGE step.

Mazel!