Selena Gomez is back on the Bieber train ... clutching and grabbing onto him on Monday night after a day of Malibu fun.


Our video tells the story ... Justin and Selena partied together at Bootsy Bellows --at a birthday celebration for a friend -- and it appears she never left his side.


Earlier in the day ... Selena was riding bitch with Justin on his Cam Am Spyder 3-wheeler as they cruised along the beach with a group of friends. You may recall ... Bieber Instagrammed last week, "Our love is unconditional."