This is HILARIOUS! Justin Bieber is finally addressing his random Twitter challenge to Tom Cruise to have the 56-year-old Mission Impossible actor meet him in an MMA ring ... and he says he was just joking!

Looks like the joke's on us -- as the Twittersphere slammed Justin for making such an outrageous challenge, he's now telling us he was just trying to "start some sh*t." You've gotta watch this -- HIGHlarious!