Ashton Kutcher -- Mastering Brazilian Jiu-Jitsu ... I'm a Blue Belt, Bitches!

Charlie Sheen should tread lightly ... 'cause Ashton Kutcher has just earned his advanced blue belt in Brazilian jiu-jitsu -- meaning, the guy can fight.

Kutcher has been training with MMA badass Rigan Machado ... and last night in Beverly Hills, the "Two and a Half Men" star was promoted to a "two stripe blue belt" -- which means he's spent hundreds of hours on the mat learning all sorts of new techniques.

Also in the house ... UFC fighters Josh Barnett and Cub Swanson. It appears Barnett was promoted to an instructor level.

Still, Kutcher is hardly the most dangerous sitcom star -- that honor probably goes to Ed O'Neil ... who's a frickin' BLACK BELT!!!

In fact, Rorion Gracie once described Ed as "As tough as they come."