What is the title of J.K. Rowling's first novel written for an adult audience?

J.K. Rowling's first novel written for an adult audience is The Casual Vacancy.

Published by Little, Brown and Company in 2012, The Casual Vacancy is a standalone novel set in the fictional English town of Pagford. Its plot begins after the unexpected death of parish councillor Barry Fairbrother, leaving a vacant seat on the local council.

Unlike the Harry Potter books, the novel does not use fantasy or a young-adult adventure structure. It examines class, local politics, family conflict and social inequality through several residents of Pagford. The book was marketed as Rowling's first adult novel, although adult readers had also formed a large part of the Harry Potter audience.

Rowling later wrote crime fiction under the pseudonym Robert Galbraith. That series began with The Cuckoo's Calling in 2013, so it is not her first adult novel chronologically.