Martha Stewart Threw 'Modesty Out the Window' for Sultry 'Sports Illustrated Swimsuit' Cover (Exclusive)

Stewart joins Megan Fox, Kim Petras and Brooks Nader as this year's Sports Illustrated Swimsuit issue cover stars

Martha Stewart does it all — and now she's a Sports Illustrated Swimsuit issue cover model!

The lifestyle mogul, 81, is one of four cover models for this year's magazine alongside Megan Fox, Kim Petras and Brooks Nader. It's a gig that Stewart calls "once in a lifetime." She jokes, though, that the timing is curious.

"It should have been 30 years ago," Stewart tells PEOPLE exclusively with a laugh. "That would have been great. It was not a goal I set for myself, but once asked, I thought to myself, let's do it!"

Stewart, who wears a variety of swim looks in the magazine, which hits newsstands on May 18, says a bathing suit spread like this was meaningful to her — and something she sees as meaningful for any woman.

Related:Martha Stewart Reveals Her Secrets to 'Looking Amazing' with Glowing Selfie of Her New Haircut

"It's important to stand up to challenges, whether they're beauty challenges or intellectual challenges, even philanthropic challenges," she shares. "To be able to stand up and do it — to me — is so fun! And this is accomplishing something that not everybody would accomplish."

The spread was shot by photographer Ruven Afanador, who also took photos of the businesswoman and TV personality in the 1990s for her magazine. That history helped her feel comfortable working with the "magnificent" photographer again on this project.

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Stewart's photos find her in a variety of beautiful places in the Dominican Republic. She describes the different settings where her images were captured — all in one "long, long fashion day" — including splashing around in the water while polo ponies paraded around her and in natural backdrops that allowed her and her swimwear to steal the show.

To prepare for her glamorous day, Stewart says she upped her usual skincare and wellness routines to ensure she was feeling her best in front of the camera.

"I did Pilates three, maybe four times a week and did not have a drink — no alcohol whatsoever for two months," she says, adding that she continued getting regular facials and was very careful about what she put in and on her body (and continues to do so because it made her feel good).

"I also got a spray tan a couple days before the shoot, which isn't something I usually do," Stewart reveals. "It's nice when you're down in that azure water. I got a light tan, and it was very pretty, very natural. I'm very pleased with the pictures, I really am."

Stewart wears bathing suits by Monday Swimwear, Saint Laurent, Eres and more, and landing on the ones that would make the cut was a process that she says was exhausting but exciting.

"I tried on all the bathing suits and outfits the night before the shoot, and there were so many things to try on," she says. "You have to throw modesty out the window because they're prodding you and pulling and straightening — so I had to grin and bear it."

But if you're thinking that Stewart would be wearing chic cover-ups and more modest suits, you'd be mistaken. Stewart wasn't down for such a thing, because she was all for showing off her "good décolleté."

"They're actual bathing suits that I'm wearing — I'm not all covered up," she tells PEOPLE. "I don't like caftans. They're very glamorous shots. Probably like any of the models, I would have preferred if I could have worn a really skimpy bikini, but at my age, wearing what I was wearing was appropriate. They're not cover-up bathing suits, though, I don't like those."

"Our goal with the 2023 issue is to continue to evoke captivating and thought-provoking conversations," says MJ Day, SI Swimsuit editor-in-chief, adding that Stewart is a "legend."

"While the industry wavers on its arbitrary notion of beauty, our issue has stayed the course, showcasing the women of today, the women shaping the future. This year, we're featuring an extremely diverse group of women starting with our cover models, who are collectively the most unapologetic women from different walks of life who continue to forge their own paths on their own terms."

Just like in previous years, the SI Swim cover stars were kept completely under wraps until Monday's big reveal. In fact, Stewart tells PEOPLE that she didn't know who the other three cover stars were. She was sworn to secrecy on her own role in the magazine, admitting that she told very few people about the shoot, though did share the news with her daughter Alexis.

RELATED VIDEO: Martha Stewart Shares Salon Selfies Highlighting Her 'Great' Skin: 'Unfiltered. No Facelift'

"I was absolutely chastised!" Stewart says of being told she couldn't tell anyone about her cover. "I can't do this, I can't do that," she adds with a laugh. On the long list of people kept in the dark about her photos is close friend Snoop Dogg, whom Stewart thinks will be chuffed.

"He'll probably say it's cool," she says, adding, "Or he'll say, you should have kept your shirt on."

At the end of the day, Stewart is grateful for the Sports Illustrated team putting her on the cover as such an iconic face of sexiness at any age. "They're selling bathing suits, they're selling travel, they're selling beauty, they're selling sex," Stewart says. "I think we accomplished that."

See all of Stewart's Sports Illustrated Swimsuit issue photos on the magazine's website now, as well as all of the other models featured in this year's issue. The magazine will be on newsstands on May 18.

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