What is the name of the headquarters of the Order of the Phoenix in J.K. Rowling’s novels?

The headquarters of the Order of the Phoenix is 12 Grimmauld Place.

The London townhouse serves as the Order’s headquarters in Harry Potter and the Order of the Phoenix. It is the ancestral home of Sirius Black and is protected by powerful concealment magic. The address appears to be hidden between ordinary buildings, making it inaccessible to people who are not shown the location by the Secret-Keeper.

The house is inherited by Harry after Sirius’s death, although it remains associated with the Order’s work. Readers sometimes confuse it with Privet Drive, Harry’s non-magical childhood home, or with Hogwarts, where much of the series takes place. Grimmauld Place is especially important as a meeting point during the fight against Voldemort.