Jason Giambi is a professional baseball player for the Colorado Rockies, Number 23 on the team, who appeared in the episode "Up the Down Steroid". In the episode, he presented the award at the Special Olympics, along with Barry Bonds and Mark McGwire. All three athletes have been linked or allegedly linked to the use of steroids in Major League Baseball. He has yet to make another appearance on South Park.
Jason Giambi appears to be physically in shape and it is implied that he has large upper-body muscles due to steroid use. In "Up the Down Steroid", he wears a black and white baseball uniform which consists of white pants with black stripes, a white baseball jersey with black stripes, and a black hat with the letter 'G' on it in white. He also wears a black belt.