The rapper was the hired talent Sunday at Mecedes-Benz Stadium ... but weirdly enough, he didn't put on a halftime performance -- but rather, did a quick set after the third quarter ... which definitely took everyone by surprise.

The reason ... the dude literally rappelled down onto the stage below from way up toward the top of the facility -- and he did it kinda slowly, partially rapping a song in the air.

Looks like he was doing 'Move Bitch' on his way down ... and he was hyping up the crowd too, who ate it up. Unclear if he did a whole set once he touched down or not (there was a game going on, after all), but he got a little shine regardless.

The whole point of this was to celebrate 50 years of hip-hop in the ATL ... of which Luda has been a huge contributor over the years -- not to mention countless other acts from the area.