Slytherin is one of the four Houses at Hogwarts School of Witchcraft and Wizardry. Founded by Salazar Slytherin, the house is composed mostly of pure-blood students, due to its founder's mistrust of Muggle-borns. The house is traditionally home to students who exhibit such traits as cunning, resourcefulness, and ambition. Its emblematic animal is a snake and its colours are green and silver. The house ghost is the Bloody Baron.

Slytherin corresponds roughly with the element of water with serpents being commonly associated with the sea and lochs in western European mythology as well as serpents being physically fluid and flexible animals.The colours also correspond with waters around lakes and lochs often being green, and silver being often associated with grey rain water.

Slytherins tend to be ambitious, shrewd, cunning, strong leaders, and achievement-oriented. They also have highly developed senses of self-preservation. This means that Slytherins tend to hesitate before acting, so as to weigh all possible outcomes before deciding exactly what should be done.

The Sorting Hat claims that blood purity, is a factor in selecting Slytherins. This suggests that it bases its decisions more upon the views of its members than on any accurate test of a student's lineage. Muggle-born Slytherins exist, but are very rare, as noted insultingly by Scabior the Snatcher.

Contrary to the wishes of the other founders of Hogwarts School of Witchcraft and Wizardry, Salazar Slytherin wanted a stricter policy of admission to the school —one limited strictly to pure-blood students. The controversy this created apparently led to him leaving the school and the Chamber of Secrets behind. A millennium later, the house's students are still considered affiliated with Salazar's reputed views, and open expression of pro-blood purity sentiment does seem to be more common in Slytherin than in other Houses.This disagreement over blood purity, which caused Salazar Slytherin's friendship with Godric Gryffindor to break, is the origin of the feud between Slytherin and Gryffindor, the two most competitive of the houses..

Slytherin's head of house for Harry's first six years of school was Severus Snape; however, as he fled with the Death Eaters in 1997, Potions master Horace Slughorn became the head of Slytherin House. Upon Snape's return as Headmaster later that year, he apparently approved of Slughorn's appointment, or at least did not assign any Death Eaters as a new head of house.