Because "Vikings." That's why.

The NY Mets pitcher -- who earned the nickname Thor because of that godlike blonde mane -- was offered a role on the insanely violent History Channel show but it came with a catch ... ya gotta tweak that 'do.

Don't worry, it's not all the way gone -- as Noah put it, "Party on top, business on the sides."

Noah says his character's name is "Thorbjorn" -- he's a badass -- and he'll be featured on season 6. Chances Thorbjorn meets a gruesome death? Very high (everyone dies on that show).

Producers must be stoked Noah actually showed up for the shoot -- as we previously reported, they've been screwed over by big celebrity guests in the past.

In fact, Katheryn Winnick -- who plays Lagertha on the show -- says Conor McGregor bailed on filming a scene at the last minute ... forcing Katheryn and her team to rewrite the episode.