The Half-Blood Prince is Severus Snape.

In the story, Harry discovers an old Potions textbook filled with handwritten notes and spells signed by the mysterious “Half-Blood Prince.” Near the end, Snape confronts Harry and explicitly reveals that he himself is the Half-Blood Prince, a name he created from being a half-blood wizard (Muggle father, witch mother) and his pure-blood mother’s surname, Prince.