Lord Voldemort (born Tom Marvolo Riddle) is the main antagonist of the Harry Potter series written by British author J. K. Rowling. Voldemort first appeared in Harry Potter and the Philosopher's Stone, which was released in 1997. Voldemort appeared either in person or in flashbacks in each book and film adaptation in the series, except the third, Harry Potter and the Prisoner of Azkaban, where he is mentioned.

In the series, Voldemort is the archenemy of Harry Potter, who according to a prophecy has "the power to vanquish the Dark Lord". Almost no witch or wizard dares to speak his name, instead referring to him by epithets such as "You-Know-Who", "He-Who-Must-Not-Be-Named" or "the Dark Lord". Voldemort's obsession with blood purity signifies his aim to rid the wizarding world of Muggle (non-magical) heritage and to conquer both worlds, Muggle and wizarding, to achieve pure-blood dominance. Through his mother's family, he is the last descendant of wizard Salazar Slytherin, one of the four founders of Hogwarts School of Witchcraft and Wizardry.