Check out Mike Beltran -- one of the top referees in MMA -- turning his legendary facial hair into a coronavirus mask ... just for fun!

Of course, Beltran -- who's reffed fights for everyone from Henry Cejudo to Demetrious Johnson and Nate Diaz -- is famous for that long, flowing mustache.

So, with the COVID-19 pandemic in full swing, Beltran decided to have fun ... and show off his version of nature's mask!

"Well, my friends, I think this works. I don't think anything will penetrate the 'state," Beltran joked.

Obviously, he's just kidding around (USE A REAL MASK, PEOPLE!) but ya gotta love it!

"Hope you guys had a good laugh at my expense," he added (assuming with a smile on his face?).