The former Disney actress, who continues to battle an eating disorder, hit out at Star for a cover story in which it claimed the wife of Prince William is "gaining mommy weight".
Tweeting the image of the slimline royal, Lovato wrote: "So apparently Kate Middleton (William?) is pregnant because of THIS picture... Are you KIDDING ME?!!
"And people wonder why girls/guys have body image issues.. Whether she is or not, she still has a beautiful body. Shame on you Star magazine."
Lovato recently accused the TV and film industries of working to "glorify an unhealthy body image" and called for them to "take more responsibility".
The 20-year-old has admitted that she occasionally relapses into her old ways since entering rehab for body image issues and self-harming in 2010.