The ballet movie - based on a true story - tells about the ambitions of a teenager who was born as a boy but wants to become a professional ballerina.
Leading actor Victor Polster also won an award, as he was chosen as best actor in the category "Un certain Regard". The movie had already clinched the so-called Queer Palme, as best one from a selection of movies with an LGBT theme playing at Cannes. The international film critics' organisation Fipresci had also bestowed a prize on the debut.
"Girl" will be shown in the Belgian movie theatres in the autumn. The last Belgian movie to win the Caméra d'Or was "Toto le Héros" by Jaco Van Dormael.
How four years of hard work are paying off now
"I feel so good. We worked very hard on this movie for four years, and everyone showed a lot of passion. To get a prize at this level is fantastic, and something I will never forget", Lukas Dhont told reporters.
The jury, headed by Ursula Meier, lauded the Belgian film for "combining delicacy and power." (photo below: 16-year-old leading actor Victor Polster).