The boxing legend is now a full marijuana advocate and entrepreneur -- and to celebrate the 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.

In between tokes, Tyson also revealed his dream celebrity smoking buddy -- explaining why he would have LOVED to spark up with Bob Marley.

"He is such an amazing man, I wish I had the opportunity to meet him."

Obviously, Bob passed away a long time ago -- but Mike says he's friendly with Marley's kids ... and even visited Bob's home in Jamaica!

There's more ... Mike is also honoring the "holiday" by posting some celeb interviews on his YouTube page including Action Bronson, Kenan Thompson and Jadakiss.

Look, some people go all out for Christmas ... other people, 4/20.