Justin Bieber, Hailey Baldwin -- We're Not Coming Up for Air (PHOTO)

Justin Bieber is owning the obvious ... he's hooked up with Hailey Baldwin.

Justin posted this pic which tells the story we told you over the weekend ... they've been in St. Barts and not hiding their relationship at all ... making out at a fancy schmancy New Year's Eve party in front of hundreds.

Selena, Kourtney and maybe Kendall ...cover your eyes.