Post Malone got emotional remembering Oliver Tree ... dedicating his concert to the late musician after a fatal helicopter crash ... and pouring out some beer for a fallen friend.

Posty took a moment during his Tuesday show in Toronto to honor his late pal, speaking directly to the crowd about Oliver's impact before pouring out some Bud Light.

It was a heartfelt tribute, with Post praising Oliver's art, heart, and soul before addressing him directly ... saying he wanted Oliver to know just how loved he was.

As we reported, Oliver was among six people killed Sunday morning when two helicopters collided above an electric vehicle yard in southwest Rio de Janeiro. The airships burst into flames and crashed to the ground.

Authorities are still investigating what caused the deadly crash.