S.I. Swimsuit Editor Strips Down to Bikini ... 'Beauty Comes In All Shapes' (VIDEO)

The woman behind the Sports Illustrated Swimsuit Edition is turning the tables on herself -- posing in her bikini to spread her message, "There is not a singular definition of beauty."

Of course, the magazine has gotten criticism over the years for showcasing women with insane physiques that could cause body image issues for people unable to look like the traditional model.

But MJ Day says the S.I. Swimsuit issue has evolved under her leadership and now features more diverse body types than ever ... body types everyone should be proud of.

So, she put her money where her mouth is ... and by the way, good for her -- she looks great!