Ben Affleck is leaving no doubt -- he and 'SNL' producer Lindsay Shookus are in a relationship -- but someone needs to throw in the towel, because as you see from Ben's shirt, L.A.'s super humid.

The couple stepped out for pizza Monday night in the Pacific Palisades. They were also spotted leaving a coffee shop Monday morning. 

As we reported Ben had moved on and started dating someone back in April when Jennifer Garner filed for divorce ... looks like Lindsay was that woman. The divorce should be final in a few months. We're told it's completely amicable.

Ben's clearly off the market.