Showbiz
© Rex Features
The 22-year-old model, who is married to tennis star Andy Roddick, was revealed as the cover girl during last night's Late Show With David Letterman.
A preview of the magazine shows her posing in a yellow bikini and smiling.
Writing on her Twitter account, she said: "The issue is amazing, glowing, fun, the girls are happy... it's not over-the-top, the girls and editors should be so proud."
She later added: "Craziness! Really can't believe this is all happening!"
Roddick added on his own account: "So happy, proud of my wife... the new cover for Sports Illustrated swimsuit issue! Unreal! So excited!"
More: Brooklyn Decker, Showbiz
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