It was previously reported that Faris was under consideration for the female lead in the Sacha Baron Cohen comedy.
However, according to The Hollywood Reporter, Faris has won out over Saturday Night Live's Kristen Wiig and Community's Gillian Jacobs for part.
The Scary Movie actress will play the owner of a Whole Foods-type store in New York City.
The film will be directed by Larry Charles, who also helmed Cohen's Borat. It is described as "the heroic story of a dictator who risked his life to ensure that democracy would never come to the country he so lovingly oppressed."
Cohen will star as both a goat herder and a disgraced foreign ruler who gets lost in the US. Ben Kingsley, who recently signed on for the movie, will play one of Cohen's subjects.
The Dictator is scheduled to open in US and UK cinemas on May 11, 2012.