NBA Star Joe Smith -- GETTIN MEDIEVAL ... In Rap Attack on Sterling

Ex-NBA star Joe Smith -- the #1 overall pick in the '95 Draft -- is spittin' hot fire at Donald Sterling ... hitting the disgraced Clippers owner with a brand new rap verse ... and TMZ Sports has the footage.

Ever since Smith called it quits from the NBA in 2011, he's been focusing on pursuing a rap career -- and even started his own label, Nova Unit Records. He's also got a rap name, "Joe Beast."

So, when we saw the former Lakers player out in L.A. yesterday -- he flexed his skills by droppin' a verse about the NBA's biggest villain ... complete with a dis involving the most famous sorcerer of all time ... and it's pretty damn awesome.