Macmillan is the surname of a pure-blood wizarding family, and one of the Sacred Twenty-Eight. As of the 1990s, the family is comprised of nine consecutive generations of pure-bloods - as, through Melania Black, née Macmillan, they are distantly related to the Black and Prewett families.
Recent generations of Macmillans may be considered "blood traitors", since Ernie Macmillan had friends of other blood statuses and fought against Death Eater ideals as a member of Dumbledore's Army. However, the Macmillans are seemingly more respectable in pure-blood circles than a family such as the Weasleys, since Melania Macmillan married into the highly prejudiced House of Black..
- Ernie Macmillan — a Hufflepuff student in Harry Potter’s year at Hogwarts School of Witchcraft and Wizardry
- Melania Black née Macmillan — wife of Arcturus Black III, paternal grandmother of Sirius Black