The wizard prison guarded by Dementors in J.K. Rowling’s Harry Potter novels is Azkaban.
Azkaban is an island fortress in the North Sea, used by the British wizarding authorities to imprison dangerous criminals. Dementors serve as its guards, making the prison especially feared because they drain hope and happiness from nearby people.
The prison first appears by name in Harry Potter and the Prisoner of Azkaban, the third novel in the series. Sirius Black’s escape from Azkaban is a major part of that book’s opening mystery, although the story later reveals that he was imprisoned unjustly.
Azkaban is sometimes confused with Nurmengard, another wizarding prison associated with Gellert Grindelwald. Nurmengard is located in continental Europe and has a different history, while Azkaban is the prison operated by the British Ministry of Magic.