"OMG @samsmithworld you are too skinny!! Stay healthy!!! pls!," one fan commented on the picture of the 23-year-old, who has been dropping weight over the last several months.
The photo, captioned, "California daze", showed the "Writing's On The Wall" singer with a buzzed haircut and a slim figure, reported Fox News.
One fan commented, "HOW DIFFERENT DOES HE LOOK whaaat."
But others stuck up for Smith's look, with one commenter writing, "You look great. Don't listen to anyone who says you need to be thicker or thinner. You're perfect when you're happy."
"Three weeks ago I met a woman who has completely changed my life," he wrote.
"Amelia Freer has helped me lose over a stone in 2 weeks and has completely transformed my relationship with food.