Miley Cyrus -- POSES NUDE ... Covers Junk

It was only a matter of time -- Miley Cyrus went FULL NAKED for an artsy photo shoot ... but sorry pervs, it ain't exactly a Penthouse spread.

20-year-old Miley stripped down to her birthday suit for a Marc Jacobs t-shirt -- which is part of the designer's "Protect the Skin You're In" campaign to raise awareness for melanoma research.

Of course, the slogan is carefully placed over Miley's chest to cover her nipples ... and she uses her hand to hide her hooha. You can breathe easy, Billy Ray ...

Miley posted the image on her Twitter page -- along with the caption, "Ts are available at 9 Marc Jacobs boutiques including SanFran, LA, Chicago, New York, Boston, & Savannah GA!"