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The Tree of Life actor has raised a large amount of funds and awareness for the country through his J/P Haitian Relief Organisation since the 2010 earthquake.
Haiti foreign minister Laurent Lamothe awarded Penn the job at the Cinema for Peace benefit in Beverly Hills, California on Saturday.
Penn told the crowd: "I do accept. [It would be a good] change being called, 'Hey you [expletive], to hey Mr Ambassador!"
George Clooney, Julia Roberts, Demi Moore, Orlando Bloom, Mel Gibson and Hilary Swank were among the celebrities in attendance at the charity event. They helped raise $5 million (£3.1 million) in charity funds through a private auction.
Damien Rice reportedly paid $475,000 (£296,875) for a guitar owned by Bono, while Julia Roberts's husband Danny Moder bid $50,000 (£31,250) for a trip to Haiti with Penn, according to the New York Post.
Penn's charity organisation was given a $2.25m (£1.43m) grant by the World Bank in September to continue its work to help citizens of the country.
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