Martha Stewart is proving age ain't nothin' but a number ... 'cause the 81-year-old entrepreneur is gracing the cover of the Sports Illustrated Swimsuit issue this year -- making her the oldest model to ever get the honor!!
S.I. revealed the special cover on Monday ... showing the best-selling author and TV personality wearing a white one-piece halter bathing suit in the Dominican Republic.
Stewart explained her decision to pose for the famous mag ... saying, "When I heard that I was going to be on the cover of Sports Illustrated Swimsuit, I thought, 'Oh, that’s pretty good, I’m going to be the oldest person I think ever on a cover of Sports Illustrated,'" Stewart said.
"And I don’t think about age very much, but I thought that this is kind of historic."
Stewart -- who turns 82 in August -- added she wanted to do the SI Swimsuit issue to prove you can look and feel good at any age.
"Age is not the determining factor in terms of friendship or in terms of success, but what people do, how people think, how people act, that’s what’s important and not your age," Martha said.
Stewart was one of four women picked to appear on the cover -- Megan Fox, Kim Petras and SI Swimsuit veteran Brooks Nader also got the nod.
FYI, this isn't Stewart's first modeling gig ... she modeled back in her teenage years for several companies, including Chanel.
The issue -- which also features LSU athletes Angel Reese and Livvy Dunne -- drops later this week.