Director Quentin Tarantino headed the jury which unanimously chose Coppola's film as the best movie at the 11-day annual festival.
"This film enchanted us from its first screening," Tarantino said.
Coppola has described the film as a "portrait of today's L.A."
"Somewhere" tells the story of a movie star, played by Stephen Dorff, who comes to see the emptiness of his existence through the eyes of his 11-year-old daughter, played by Elle Fanning.
With "Somewhere," Coppola, who herself grew up in the world of film thanks to her father, director Francis Ford Coppola, lampoons Hollywood's star treatment.
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