We've heard of boyfriend jeans, but stepson tees? That could become a new trend, if women follow Megan Fox's lead.

While celeb moms like Madonna have been known to steal pieces from their daughter's closets, Fox has taken to transforming her wardrobe by borrowing T-shirts from hubby Brian Austin Green's 8-year-old son, Kassius.

And she's been doing it for years, according to the Daily Mail!

Earlier this month, the "Jonah Hex" star showed off her taut tummy -- and a taut top -- in Kassius's green "Voltron: Defender of the Universe" shirt.

She also bared her belly back in 2008, proving herself a force to be reckoned with in the boy's snug white "Star Wars" tee.

You gotta hand it to Fox -- she sure knows how to make being a sci-fi nerd look sexy. And if Kassius were smart, he'd put those Fox-worn shirts on eBay, pronto!

In related news, Fox was just named the new face of Giorgio Armani Cosmetics.