Scott and Justin Bieber's ex-gf got super cozy on a yacht in Monaco Sunday. He seemed to be letting his hands do the talking ... to her bikini-clad ass ... while they lounged on a daybed.

As we reported, Scott's run afoul of the Kardashian sisters with his revenge mission against Kourtney. The Chantel rendezvous won't help that situation.

She's been friendly with Kylie Jenner ... so, we have no doubt his name will come up in a convo between them sometime soon.

New day, new chick? Over the weekend, Scott was seen getting flirty with Sofia Richie. She's already denied there's anything going on between them, but she has been hanging with Chantel over in Monaco.

Sofia took a pass, but Chantel looks like she's all in.