Ben Affleck and Jen Garner are NOT getting back together romantically ... despite speculation and reports claiming the contrary ... TMZ has learned.

A source close to Ben tells TMZ ... headlines suggesting BA is interested in rekindling a romantic relationship with his ex-wife are "ridiculous" gossip. We're told there is no interest from Ben and he's happy they can be good friends in a great co-parenting situation.

This hammer-drop response comes after multiple reports surfaced suggesting the former couple was potentially getting back together as a romantic duo ... which, again, ain't the case. Our sources tell us Ben is NOT currently dating and it's not even of interest to him right now.

We're told Ben's in the middle of directing a new movie and is focused on spending as much time as he can with his children he shares with Jen.

The speculation and reports began after the Hollywood exes spent quality family time together on Sunday for a paintball birthday party for their son, Samuel. In the video, you see Ben with his hands around Jen ... hugging her from the side as she shoots her paintball gun.

Our sources say the exes do a lot of activities together with their kids that the public doesn’t always see. To Ben and Jen, we're told Sunday's outing is nothing new and it's ridiculous to speculate there's anything more than a friendship.

In case you didn't know ... Garner is in a relationship with investment business CEO John C. Miller.