The singer-songwriter stepped out with the model-turned-mogul and hit up the popular Ferdi restaurant in the city ... and, check out the pics 'cause there is a whole lot of PDA happening here.

Bad and Kendall wear matching gray outfits -- him in a stylish suit and Kendall clad in a skintight bodysuit ... and, clearly the temptation's too great for BB 'cause he's got hands all over Kendall's booty as they leave the hot spot.

KJ doesn't seem to mind though flashing a huge smile while hanging out with her on-again, off-again man. The two reportedly then went to an exclusive nightclub in Paris ... one that attracts the A-list's biggest and brightest stars.

The pair's night out in Paris is another big step in their slowly rekindling relationship ... remember, we saw the two on a romantic date in Puerto Rico just a couple weeks ago -- when Kendall seemed to be gazing lovingly into Bad's eyes.

At the time, it seemed clear their relationship was back on ... and, these new PDA-filled pics are certainly adding more fuel to that fire.

As you know ... the two broke up in the winter, but have regularly been spotted together in the months since. They hung out after the Met Gala, Kendall went to his concert in Orlando and they later grabbed dinner in Miami -- so, there's always been unfinished business here.

BTW ... Kendall's in Paris for fashion week -- so, seems like she's doing a fair balance of work and pleasure here.