Chazelle beat Denis Villeneuve (Arrival), Mel Gibson (Hacksaw Ridge), Kenneth Lonergan (Manchester By the Sea) and Barry Jenkins (Moonlight).
Chazelle's win makes him the youngest director to win an Academy Award. The previous youngest was Norman Taurog, who won for "Skippy" at the age of 32.
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The musical stars Emma Stone and Ryan Gosling in lead roles, and follows a musician Sebastian, and an aspiring actress Mia, who meet and fall in love in Los Angeles.
"La La Land" is Chazelle’s third film as a director. He made his directorial debut in 2009 with "Guy and Madeline on a Park Bench," and in 2014, he returned with "Whiplash."