She wears a tiny red-and-brown, sparkle-covered string bikini with a view of the waters of Australia behind her as she poses for the magazine cover, which was unveiled on TV Monday night on "Late Show with David Letterman" on CBS.
"That was one of the first photos from the first shoot," she said in a telephone interview before the big reveal. "It was my favorite swimsuit. ... I felt sexy in it but confident."
Upton, 5 feet 10, was born in Michigan but raised in Florida. Her hometown is Melbourne, on the state's east coast.
"In Florida, people walk around in their flip-flops, bikinis and jean shorts, so I'm very comfortable in a bathing suit," she said.
The 2012 magazine cover is her second time appearing in the annual SI issue that's the equivalent of the Super Bowl for models. Last year, when Irina Shayk was on the cover, Upton was chosen as rookie of the year.
"I have had a great year," she said. "For me, this is groundbreaking."
A huge perk of the job as the official swimsuit model is visiting lovely beaches, including shoots for the magazine in Sydney and Cairns, Australia, and Apalachicola, Fla.
She said living a healthy lifestyle for her includes working out regularly with a personal trainer and watching what she eats. But there's no special diet to prepare for a bikini shoot, she said, and she admits to splurging on the occasional ice cream or plate of fries.
Christie Brinkley and Elle Macpherson are among the now celebrity models who previously had this gig. Upton said, "I am so glad to be in their company."