Rumour has it - well, this is my theory - the guy is that much of a unit that he simply couldn't find a shirt to go over those ridiculously muscly arms of his. Don't quote me on that though.
But if, on the other hand, he simply couldn't be bothered with dressing up all fancy, who can blame him? I imagine it gets a bit stuffy in there with all those famous types - and when you're Jason Momoa, you can pretty much do whatever you want.
Either way, his tank top - or 'sweater vest', if you're reading this in the US - definitely attracted a lot of attention, with people taking to Twitter, as usual, to voice their opinions - which were unequivocally supportive.
Momoa can literally do no wrong in the eyes of the world and more power to him.