NBA Star Russell Westbrook -- I'm An Underwear Model Now!

Russell Westbrook doesn't want your penis to suffocate ... and that's why the Oklahoma City Thunder star just signed a deal to model a high end underwear line that prides itself on "wiggle room," TMZ Sports has learned.

Our well-placed underwear sources tell us ... Russell inked a huge deal with Kings and Jaxs that will require him to not only model the undies, but will also give him creative and design input.

FYI --K&J prides itself on how the underwear is built to let your man parts breath -- the web site says, "The little man doesn't like to be restricted with little or no oxygen."

As for Russell, he's already posed for several pics in the undies -- some without a shirt -- and dude's jacked.