The TV presenter and businesswoman was chosen as one of four women for the magazine’s annual swimsuit edition alongside Megan Fox, Kim Petras and Brooks Nader.
She wrote on Instagram that she was “thrilled” to be on the cover.
“My motto has always been: ‘when you’re through changing, you’re through’, so I thought, why not be up for this opportunity of a lifetime?
“I hope this cover inspires you to challenge yourself to try new things, no matter what stage of life you are in. Changing, evolving, and being fearless – those are all very good things, indeed.”
Stewart told the magazine: “When I heard that I was going to be on the cover of Sports Illustrated Swimsuit, I thought, Oh, that’s pretty good, I’m going to be the oldest person I think ever on a cover of Sports Illustrated.
“And I don’t think about age very much, but I thought that this is kind of historic.”
The photoshoot took place in the Dominican Republic with photographer Ruven Afanador.
Sports Illustrated Swimsuit’s editor-in-chief MJ Day praised how Stewart had never “let her circumstances dictate her outcome”.
“She’s changed with the times – always one step ahead, it seems – to build a wide-reaching business empire,” she said.
The magazine called Stewart the “OG of influencers”; she reaches more than 100 million fans monthly with her magazines, television shows and books.