Demi Lovato is becoming a much more dangerous person -- she just earned her blue belt in Brazilian Jiu-Jitsu ... after training her ass off for the past year.
The blue belt is one level above white belt -- and 3 levels away from being a black belt.
The singer was awarded her new rank Monday at Jay Glazer's Unbreakable Gym in L.A. -- where she's spent the past 12 months grappling with MMA superstars like Chuck Liddell and Randy Couture.
Glazer -- who owns the gym -- flew back to town specifically to present Lovato with the new belt and tweeted, "I'm more proud of the work [Demi] puts in with nobody watching than i am what she does with the world watching.