Justin Bieber & Selena Gomez — Secret Wedding Trip to Cabo

Justin Bieber and Selena Gomez snuck down to Mexico yesterday for a very private wedding — but put away the pitchforks, Bieber fans … they’re not the ones tying the knot.

TMZ has learned … the duo flew south of the border to attend the wedding of one of Selena’s friends Shannon Larossi — a costume designer Selena met on the Disney show “Wizards of Waverly Place.”

Selena’s one of the bridesmaids — and we’re told, she even plans to sing at the nuptials.

As for Justin — we’re told he’ll just be a regular super-famous guest … not a performer.