Wayne -- who was part of Floyd Mayweather's entourage -- was leaving the ring at the MGM Grand when in the chaos that ensued following the fight, someone threw a bottle that struck the rapper on the shoulder.
Lil Wayne tried to FIGHT Marcos Maidana's trainer ... moments after the Mayweather fight ... and sources close to the situation tell TMZ Sports it was all over a flying water bottle.
We're told ... Wayne believed the bottle-thrower was a member of Maidana's entourage (the fighter's trainer) -- who was following close behind -- and Weezy immediately tried to confront the guy to fight it out.
You can see in the video ... a member of Wayne's entourage grabbed the rapper and kept him from getting into it ... though it's clear, Weezy was heated.
At one point, you can hear Wayne scream at Maidana's camp, "Let's GO, n***a!"
Ultimately, both sides were separated without further incident.