The actor just posted a new photo of himself on the 'Gram -- a shirtless one, at that -- and he's showing off some new artwork on his back. Specifically, it's a tattoo that screams surfer vibes ... but it's really nothing compared to what's atop his noggin in the pic.

Check it out ... Jonah's tat reads "Body Love" and it's got a "rock on" hand sign attached (in color, too, BTW). JH seems proud of the new ink -- but he's kinda ignoring the headline ... namely, that his hair is now platinum blonde, beard too for that matter. Looks like he's been baring his blonde locks for a quick minute ... since at least early July, apparently.

Jonah's got a few different photos of him in blonde on his IG page -- and, boy, is it something.

He looks like a wild man of the water, frankly, which is appropriate ... because the dude HAS been shredding the gnar in recent years -- hitting up Malibu and other remote surf spots in and around SoCal to get his wave-riding on. He seems to be getting pretty good too.

Oh, and we know what you're thinking ... is this new look for a movie or something??? That doesn't appear to be the case ... Jonah doesn't have any roles coming up (that we can see) which would require him to rock this sort of 'do/vibe. So, it seems to be all him ... and au naturel.