Banksy isn't so elusive in the time of coronavirus -- the guy's out and about in the open, tagging up public trains in London and his message is clear ... wear a damn mask!

The mysterious graffiti artist -- who's known for some of the most elaborate paint jobs and publicity stunts around -- pulled an awesome prank across the pond while riding London's Underground.

Check it out ... Banksy documented his latest work in an awesome vid, showing him infiltrating a busy train and getting to work beautifully tagging it.

You can't really see his face all that well -- he has a face mask and full PPE on to conceal his identity -- but it's wild that Banksy surfaced so overtly here.

He fooled the riders into thinking he was there to do business, and nobody gave him a hard time when he whipped out his paint can. Banksy proceeded to paint his signature sewer rats, only this time with a heavy COVID theme.

Some of them were wearing masks, some were sneezing snot across the windows -- and of course, Banksy added some pop culture commentary by invoking a Chumbawamba hit that you haven't heard in years.

Watch ... it all makes sense in the end.