Ben Affleck is getting hands on with Jennifer Garner ... pulling his ex in for a tight hug at their son's birthday party.

The Hollywood exes spent some quality family time together Sunday ... skipping the Oscars for a paintball birthday bash for their son Samuel.

Ben's got his hands around Jen in a few of the photos ... hugging her from the side as she shoots her paintball gun.

The optics are pretty interesting -- it looks like Ben is super into Jen ... and folks who were there say Ben flirted with his former flame throughout the outing.

Thing is ... our sources have repeatedly stressed Ben and Jennifer are just friends despite his recent divorce from Jennifer Lopez ... and we've been repeatedly told Garner is still in a relationship with investment business CEO John C. Miller.

But, based on what Ben's doing here, that would all seem like news to him.

The duo's always spent time together since 2018 divorce, co-parenting their kids and being there for each other in times of need ... like January's L.A. wildfires.

Ben clearly had a great time with Jen -- we previously showed you photos of him smiling all lovingly at Jen ... and now we see him hugging her.

Old love never dies?!?