Brad Pitt and Angelina Jolie's son Knox's 17th birthday kicked butt -- literally -- because he spent his big day winning a martial arts competition.
With Angelina and his twin sister Vivienne cheering him on from the crowd, Knox won the IKF Point Muay Thai Technical event on his birthday weekend.
Video from the competition shows Knox beating the crap out of an opponent ... landing a bunch of punches and kicks as they battle on a red and blue mat.