The publication made the huge announcement Monday morning ... revealing the 23-year-old would be joining rap star Megan Thee Stallion and transgender actress/model/activist Leyna Bloom as a cover model for the 2021 issue.

The decision was easy for S.I. -- Osaka is no stranger to the suit game (she has her own collection with Frankies Bikinis) ... and is one of the biggest names in sports.

Osaka also made headlines this year by advocating for mental health awareness for athletes ... famously pulling out of the French Open and Wimbledon after revealing her struggles with depression.

The current #2-ranked female player in the world is now set to compete in front of her hometown fans at the Tokyo Olympics this month ... where she's a favorite to take home the gold medal.

S.I. Swim editor MJ Day spoke about the magazine's cover models ... saying, "If there's one thing that our cover models have in common, it's that they don't have one thing in common."

"They look different, have different upbringings, have different passions and inspirations. But each is a reminder that beauty comes in many forms."

"We celebrate Naomi for her passion, strength and power geared toward consistently breaking barriers when it comes to equality, social justice and mental health."

The other two models are also making history -- Meg is the first rapper to be featured on the cover, while Bloom is the first transgender model to get the honors.

The issue also includes several sports WAGs like Olivia Culpo, Camille Kostek, Kate Bock, Kristen Louelle and Christen Harper ... as well as other S.I. vets like Brooks Nader, Jasmine Sanders, Danielle Herrington and Josephine Skriver!!

Oh, and if that's not enough ... singer Tinashe is also in the mag!!