It's 4/20 ... and you've NEVER seen Mike Tyson this happy!!
The boxing legend is now a full marijuana advocate and entrepreneur -- and to celebrate biggest weed holiday of the year, he fired up a joint and explained to TMZ Sports why the day is so important to him.
Of course, 53-year-old Mike launched his "Tyson Ranch" weed biz a while back -- and now swears by THC ... saying he views it as "medicine" and raves that it's changed his life for the better.
Now, he views 4/20 as an important day -- because it symbolizes the beginning of the "revolution" to de-criminalize weed all over the world.