The exes were cozied up in Sean's car Wednesday night outside a recording studio in Los Angeles. For the record, Ariana was at the studio first, Big Sean showed up after ... and she came outside for the reunion.

Ariana showed off one of her pooches to her ex-bae ... great excuse to get closer. BTW, they left together in his ride.

You'll recall ... Ariana and Big Sean dated for almost a year before she pulled the plug on the relationship in April 2015. She claimed the rapper embarrassed her with his "billion dollar p***y" lyric.

They both eventually moved on -- Big Sean with Jhene Aiko ... but it appears old flames die hard.

Fast forward to last winter ... Ariana dropped her "Thank U, Next" song. The opening lyric goes, "Thought I'd end up with Sean but he wasn't a match."

But, in the music vid, her "Burn Book" shows she's still got it bad for Sean ... writing Sean's "so cute, so sweet" and "(could still get it)."

And, almost 3 months later ... it looks like Sean's getting it.