From the Pulitzer FAQ:
Only U.S. citizens are eligible to apply for the Prizes in Letters, Drama and Music (with the exception of the History category in Letters where the book must be a history of the United States but the author may be of any nationality). For the Journalism competition, entrants may be of any nationality but work must have appeared in a U.S. newspaper published at least once a week, on a newspaper's Web site, on an online news organization's Web site, and in some categories, in a U.S. magazine published at least weekly or its Web site.
http://www.pulitzer.org/page/frequently-asked-questions