Baker Mayfield's An Underwear Model Now!

Baker Mayfield is not only the Browns' best backup QB ... he's clearly the team's best shirtless model too -- 'cause dude is selling underwear now, and he's killin' it!!

The Cleveland rookie announced his partnership with undie company PSD Underwear on Tuesday ... and he did it topless -- with a Tiger AND a Rolls-Royce.

The abs were out ... the underwear was exposed. It was great.

But ... what exactly are the under garments of Baker's choice?? Per the company, they're 88% polyester and 12% spandex with a "breathable athletic feel."

Plus ... they not only come with his Browns' No. 6 embroidered on them -- they also feature a "contoured sealed pouch."

Which ... if you're unfamiliar -- is VERY important for the ex-Oklahoma QB.