Shawn Mendes suffered a wicked hand injury ... and whatever he did, it was a bloody mess.

The singer-songwriter was recently snapped holding a towel over his left hand ... and it looked like it soaked up a lot of blood.

It appears he injured both his thumb and middle finger -- but if he was in pain, he didn't show it ... SM flashing a smile -- but also looking annoyed with himself.

No idea what he did to cause such a wound ... but TMZ is working to find out! Hopefully, whatever he did isn't so severe that he's gotta put down the guitar for a while!

Heal up fast, Shawn!