Which Harry Potter book contains the longest title in the original seven-novel series?

Harry Potter and the Order of the Phoenix contains the longest title in J.K. Rowling’s original seven-novel series.

The fifth Harry Potter novel was published in the United Kingdom on 21 June 2003. Its title has ten words when counted in the standard English form, making it longer than the titles of the other six main novels. The story follows Harry’s fifth year at Hogwarts and introduces the student resistance group Dumbledore’s Army.

The title refers to the Order of the Phoenix, a secret organization founded by Albus Dumbledore to oppose Lord Voldemort. It should not be confused with the film title’s word count conventions or with the length of the novel itself, which is a separate measurement and is often discussed because the book is exceptionally large.