Shayk only learned she'd won the coveted spot Monday when the cover was revealed on "Late Night With David Letterman."
"There were 10 models there to do the Top 10 List, and nobody knows until the last minute who the cover will be," Shayk said. "And then I saw it, and I couldn't believe it."
In keeping with tradition, the cover of the special issue is kept secret until the magazine is ready to hit newsstands.
Shayk, the girlfriend of soccer superstar Cristiano Ronaldo, has been in the issue five times before, but this is her first cover.
The Associated Press contributed to this report.