La Vie d'Adele – Chapitre 1 & 2 (Blue is the Warmest Colour) was chosen from a field of 20 films exploring sex, violence and emotional anguish which were vying for the Palme d'Or, one of the most coveted film awards after the Oscars.
Critics picked the three-hour film as a possible winner at the 66th Cannes festival, but wondered if its explicit lesbian sex scenes – one lasting up to 10 minutes – would deter the jury deciding the awards led by US filmmaker Steven Spielberg.
In an unusual move, Spielberg said the award would be shared between Kechiche and his two lead actresses Adele Exarchopoulos and Lea Seydoux as they were central to the film's success.
"I think it will get a lot of play … I think this film carries a very strong message, a very positive message," Spielberg told journalists.
"It was the perfect choice between those two actresses and this incredible very sensitive and observant filmmaker."
Spielberg said he supported same-sex marriage, but downplayed any suggestion the award was to promote this cause.
Festival director Thierry Fremaux said the film was timely, as hundreds of thousands of demonstrators marched in Paris on Sunday to protest France's recent legalisation of same-sex marriage.
"Everyone who is against same-sex marriage or love between two people of the same sex must see the film," he told Reuters.
Kechiche, a Tunisia-born actor who made his directorial debut in 2000, was virtually speechless as he accepted the award, which he dedicated to the youth of France and Tunisia who, during the Arab Spring, "wanted only to live, speak and love freely".
La Vie d'Adele is an emotional tale of love and sexuality centred on 15-year-old Adele (Exarchopoulos) and her lover Emma (Seydoux) that follows the course of their tumultuous relationship.
Critics had also considered as a forerunner Inside Llewyn Davis about a struggling New York folk singer by the American Coen brothers Ethan and Joel, which was named as runner-up.
The third prize went to Japanese director Kore-Eda Hirokazu for the baby-swapping drama Soshite Chichi Ni Naru (Like Father, Like Son) while the best director award went to Mexico's Amat Escalante for his brutal look at Mexico's drug war, Heli.
American Bruce Dern (76) won a best actor award in Alexander Payne's road trip film Nebraska, beating out Michael Douglas, who was widely praised for his performance as flamboyant pianist Liberace in Steven Soderbergh's Behind the Candelabra.
French actress Berenice Bejo (The Artist) won the best actress award in Iranian director Asghar Farhadi's tense domestic drama Le Passe (The Past).
Before the ceremony, stars including Kim Novak and Laetitia Casta signed autographs and posed for photographers on the red carpet in blazing sunshine, a contrast to the festival's opening ceremony on May 15 when umbrellas took over in the rain.
The awards ended the 2013 edition of the world's largest film festival, where up to 40 000 film professionals also bought and sold titles on the bustling marketplace hidden away from the glitzy promotional circuit of parties and stunts. – Reuters