High school hoops phenom Mac McClung is already a viral sensation for his insane dunks -- but his cousin Riff Raff says he's a REAL DEAL player who will dominate the NBA.

Yeah, they're really blood-related ... and Riff Raff tells TMZ Sports he couldn't be prouder of the 18-year-old stud, who's already verbally committed to Georgetown.

In fact, the rapper says Mac's latest Instagram highlight -- a between-the-legs dunk ON A FASTBREAK (did we mention the kid's just 6'2''??) -- proves he's ready to hang with NBA players right now.

2019 NBA Draft is just 16 months away ...