Selena Gomez must be exhausted ... 'cause the chick was running all through Justin Bieber's mind during Monday night's Met Gala -- at least that's how JB made it seem on the way out.

Justin and Selena have been broken up for months, but as he left the ball -- which Selena also attended -- Biebs went out of his way to throw out a line that was loaded down with bait for his ex-gf.

Watch ... he absolutely, almost desperately, wanted to make a point about SG's dress last night.

Word on the street is she just broke up with Zedd, so uhh ... never say never?

Then again ... Justin didn't seem to be losing sleep over her while partying with Leo DiCaprio at Rihanna's after-party. #TurnUp