The 24-year-old appears on the front of the magazine's new annual Hot Issue with her arms stretched above her head showing a little armpit hair, sending some fans into a frenzy.
Many applauded the singer's au naturel look, while one critic wrote: "thought you were a millionaire just buy some wax."
Now Demi has stepped up to defend Halsey, writing: "There so much yes about this picture idk (I don't know) where to start (sic)."
It's not the first time Lovato has applauded her peers for body issues - she praised Bebe Rexha for opening up about sizeism earlier this year.
"F**king preach!!!" Demi wrote. "Love this and you for speaking your mind and using your voice!!!".
Lovato has battled body positivity issues for years and back in 2017 tweeted: "Sometimes when I'm having bad body image issue days, I remind myself that I'd rather live in freedom from my eating disorder than worry about what people think about my body."