Number 12, Grimmauld Place (a pun on "grim old place"), London is the address and name of reference to the home of the Black family, an ancient and pure-blooded line of wizards. It first appears in the fifth book. The structure of Number 12 Grimmauld Place is a Georgian terraced house.

Number 12 houses the Black family tree on a wall tapestry, and an enchanted portrait of Walburga Black, Sirius's mother. An ancient and deeply mad house-elf named Kreacher is loyal to the portrait of Mrs. Black. There are other portraits of members of the Black family, including Phineas Nigellus Black, one-time Head of the Black family and least-popular Headmaster of Hogwarts. The staircase is lined with the heads of beheaded former house-elves, which are mounted onto the walls.

Many security measures are in place at Grimmauld Place: there are anti-Apparation charms, the place is Unplottable, the whole house is under a fidelius charm and it is disguised from Muggles and other interlopers. In the seventh book, it is noted that the neighbours had long ago come to terms with the houses on their square running straight from 11 to 13. It is as secure as any magical dwelling can be and can accommodate a large number of people. For this reason, it was chosen as the headquarters of the reconstituted Order of the Phoenix when Sirius offered it to the Order. Only magical people can see it, and only if told the location by the Secret Keeper himself.

Because Sirius was incarcerated in Azkaban, the house fell into disrepair over the next several years. When he later returned to his family home in the fifth book, it was a gloomy and unpleasant dwelling teeming with dust, decay, magical creatures, and various dangers. Harry inherits the house at the beginning of Half-Blood Prince after Sirius's death, although he donates it to the Order (wanting no connection to the place where Sirius felt trapped and useless before his death).

In Deathly Hallows, it becomes a sanctuary for Harry, Ron, and Hermione while hiding from Voldemort. Harry loses the house to Voldemort when Yaxley grabs hold of Hermione when she attempts to escape by Disapparition. She drops Yaxley off at 12 Grimmauld Place, thus revealing the location to Voldemort.