J.K. Rowling used no pseudonym for The Casual Vacancy; it was published under her own name.
The novel appeared in 2012 after the completion of the Harry Potter series. Its credited author was J.K. Rowling, making it her first adult novel published under that name. The story is set in Pagford and begins with a sudden vacancy on the local parish council.
The question can be confused with Rowling’s crime-fiction career. Her Cormoran Strike books were initially published under the name Robert Galbraith, a pen name that was publicly linked to Rowling in 2013. That pseudonym was not used for The Casual Vacancy.
The novel’s publication demonstrated Rowling’s move into adult literary fiction. It deals with politics, class, poverty, and family life rather than magic. A television adaptation was later made by the BBC and HBO, premiering in 2015.