The Motion Picture Association of America gave the film the rating -- which forbids children under age 17 from seeing the film without a parent -- because of language.The MPAA will allow a PG-13 rating for a film in which the "F" word is used once, but "Bully" exceeds the limit.
The Weinstein Co., the studio behind the anti-bullying film, is fighting to overturn the decision, arguing it excludes the audience who will most benefit from seeing it.
Gerry Lopez, chief executive of AMC Theaters, called the documentary's R-rating "a mistake."
"The message, the movie and its social relevance defy that kind of formulaic, conventional thinking," Lopez said in a statement. "AMC will show this movie, and we invite our guests to engage in the dialogue its relevant message will inevitably provoke."
Nearly 300,000 people have signed a petition started by high school junior Katy Butler, who said she was a victim of bullying.
Streep and her daughter, actress Mamie Gummer, are to serve as co-hosts of a screening of the film with David Boies, one of the two attorneys who argued for overturning Proposition 8 in California, and his daughter, Mary Boies. The screening will be held in New York March 20.
Depp, a father of two, has voiced his support of the rating change, while DeGeneres has devoted time to the film on her television show and fashion designer Tommy Hilfiger is creating a T-shirt inspired by the "Bully" movie poster, the studio said.
Kelly Ripa and Anderson Cooper presided over a screening in New York last week, presented by Bing, the search engine from Microsoft. Photographer David LaChapelle has offered to donate his talents toward an advertising campaign.
Bieber joined the cause last week, telling his fans on Twitter about the movie and encouraging them to stand up for each other. He is currently working with The Weinstein Co. on how he can do more to support the film, the studio said.