PHOTOS: Martha Stewart Stuns as Swimsuit Covergirl at 81

The “Sports Illustrated” Swimsuit Issue has several covers this year . . . and one of them features 81-year-old Martha Stewart.  She’s wearing a one-piece bathing suit with a severely plunging neckline.  And she looks GREAT.

Martha did a little prep for the shoot.  She says, quote, “I didn’t starve myself, but I didn’t eat any bread or pasta for a couple months.  I went to Pilates every other day.

“I live a clean life anyway.  Good diet and good exercise and healthy skincare can do all that stuff, but it was kind of fun.”

She adds, quote, “To be on the cover [at] my age was a challenge, and I think I met the challenge.”

Martha is one of FOUR cover models this year.  The others are Megan Fox, model Brooks Nader, and trans singer Kim Petras.