Shawn Mendes, Camila Cabello Kissing Incessantly in Montreal

Shawn Mendes and Camila Cabello have taken their PDA world tour on the road ... which begs the question -- do these two ever come up for air?!?

The "Senorita" singers headed North to Montreal for more QT. When they weren't holding hands on a nighttime stroll, they were gettin' it at a small cafe.

It's been nonstop spit swapping ... sucking face in SFO, cuddling in WeHo and taking it East to South Beach.

So, will summer love blossom in the fall, or fade into the sunset? The doubters say this is all a PR move to hype their song, "Senorita," but it's looking more and more like this is the real deal.

BTW ... "Senorita" is killing it on the charts ... it's number 2 on Billboard Top 100 ... it even beat out Lil Nas X.