Joshua Jackson may have picked up a thing or two from "Karate Kid: Legends" ... we got video of him going H.A.M. in a boxing gym with a trainer.

Watch the video ... the 47-year-old actor ditched his shirt for the workout, really working up a sweat as he bobbed and weaved in the ring while throwing in some pretty impressive jabs!

He later goes to work solo on a speed bag like an old pro.

Joshua has publicly said that starring in "Karate Kid: Legends" in the role of a former boxer was a dream for him, but there's no word on whether he's training for another role or just really fell in love with boxing.

Either way ... you're looking great, Joshua!