The Undercroft

• Type: Storeroom, Chamber
• Location: Off the Entrance Hall, Hogwarts Castle
• Owner/Residents: Hogwarts School of Witchcraft and Wizardry
• Key Features: Small and dingy, contains a large, moth-eaten troll-leg umbrella stand.

The Undercroft is a small, rather dingy chamber located on the ground floor of Hogwarts Castle, situated off the Entrance Hall and opposite the doors to the Great Hall. Referred to by this name by the caretaker, Argus Filch, its primary purpose is to serve as a storage space for students' outdoor gear, such as cloaks and boots, particularly during periods of poor weather. The room's most prominent feature is a large, ugly umbrella stand made from a severed Troll's leg.

The Undercroft serves as the setting for a pivotal scene in Harry Potter and the Half-Blood Prince. While trying to discover what Draco Malfoy is up to, Harry Potter and Ron Weasley follow him from the Great Hall. When Malfoy is stopped by Professor Snape, Harry and Ron seize the opportunity to hide in the nearby Undercroft beneath the Invisibility Cloak to eavesdrop. Inside, they overhear a tense confrontation where Severus Snape questions Malfoy about his secret mission from Lord Voldemort. Snape confronts Malfoy about his failed attempts to complete the task and offers his assistance, forcefully reminding Draco of the Unbreakable Vow he made to Narcissa Malfoy to protect her son. This overheard conversation drastically deepens Harry's suspicions, confirming for him that Malfoy is a Death Eater and that Snape is actively trying to help him succeed in his dark mission. The event reinforces Harry's conviction that Snape cannot be trusted, a belief he holds for the remainder of the school year.

• An “undercroft” is a traditional architectural term for a crypt or storage room, often vaulted, located on the ground floor or below ground level of a building.
• In the film adaptation of Harry Potter and the Half-Blood Prince, the confrontation between Snape and Malfoy does not take place in the Undercroft. Instead, it occurs in an open corridor as students are leaving for a Hogsmeade weekend. (film)