Luigi's Mansion is a 2001 action-adventure game developed and published by Nintendo. The game was a launch title for the GameCube and was the first game in the Mario franchise to be released for the console; it was released in Japan on September 14, 2001, in North America on November 18, 2001, in Europe on May 3, 2002, and in Australia on May 17, 2002. It is the third video game in which Luigi is the main character instead of Mario, after Mario Is Missing! and Luigi's Hammer Toss. Players control him as he explores a haunted mansion to rescue Mario and battles ghosts by capturing them through a vacuum cleaner supplied by Professor E. Gadd.

Luigi's Mansion received a positive critical reception overall, with reviewers praising the gameplay, setting, and soundtrack, though its short length was criticized. The game sold over 3.3 million copies, and is the fifth-best-selling GameCube game of all time. It was one of the first games to be re-released as a Player's Choice title on the system. Initial fan reception to the game was mixed; however, it improved over the years and achieved a cult following. The game was followed by two sequels – Luigi's Mansion: Dark Moon (2013) for the Nintendo 3DS, and Luigi's Mansion 3 (2019) for the Nintendo Switch. A remake of Luigi's Mansion for the 3DS, co-developed by Nintendo and Grezzo, was released in 2018. The game was re-released for Nintendo Switch 2 via the Nintendo Classics service in 2025.

Luigi's Mansion's story takes place over four "areas", or sets of explorable rooms between boss fights. Players may also access a training room and a gallery at Professor E. Gadd's laboratory between areas or upon starting the game. In each area, players control Luigi to explore the mansion's rooms and hunt down the ghosts within them, acquiring keys to get through locked doors and eventually fighting a boss ghost at the end of the area. To assist him in his task, Luigi uses a flashlight and two devices supplied by E. Gadd – the Poltergust 3000, a specially modified, high-powered vacuum cleaner; and the Game Boy Horror, a mobile device designed after the Game Boy Color.

To capture ghosts, Luigi must first stun a ghost with his flashlight, revealing its heart. He then must use the Poltergust 3000 to suck them up, steadily reducing the ghost's hit points to zero, at which point they are captured. The more hit points the ghost has, the more time it takes for a ghost to be captured, giving them a chance to break free while leaving Luigi more exposed to harm. If Luigi's health is reduced to zero from being hurt by the ghosts or other accidents, the game is over. Along his journey, Luigi locates three elemental medals, each granting the Poltergust with the ability to summon and vacuum ghosts from fire, water, and ice sources and expel their respective elements to capture specific ghosts or solve puzzles. In addition to capturing the regular ghosts in the mansion, Luigi must catch "portrait ghosts" from some rooms, each requiring a condition be met to make them available for capture. Rooms are usually dark upon initial access, and Luigi hums nervously to the music, but once all the ghosts are captured, it brightens up, and Luigi pleasantly whistles the melody.

Using the Game Boy Horror's mobile phone-like functionality, players can access a map of the mansion, seeing which rooms they have visited, what doors are open, and which remain locked. When Luigi finds a key during his explorations, the Game Boy Horror automatically indicates which door it unlocks. In addition to a map function, the device allows the player to examine objects (including the hearts of portrait ghosts, revealing clues about how to capture them), read profiles of captured portrait ghosts, and keep track of any treasure that Luigi has found. Rooms will usually have treasure hidden within, which can be either coins, bills, gold bars, pearls, or gems. The treasures are typically hidden inside objects as well as chests that appear when rooms are cleared, though they can also be found from vacuuming ghosts. Luigi can gather these treasures by walking into them or vacuuming them; if a ghost harms Luigi, he will drop a few coins that he will need to recover before they disappear. After Luigi encounters a group of Boos hiding in the mansion, the Game Boy Horror can be used to find each one hiding in a room through a beeper sound and a flashing yellow light on the device, which turns red when Luigi is close to one. Boos can only be located in cleared rooms. Boos are trickier to catch, as they can plant decoys and traps within objects they can hide in that can fool the GB Horror and will escape into other rooms if they can.

Once an area is completed, all portrait ghosts are restored to their paintings by E. Gadd, which the player can view in his laboratory's gallery, at which point a result screen reveals the portrait ghosts Luigi has captured, along with the total amount of treasure he recovered for that stage. Once the final boss is defeated, the player is given a rank (A to H) after the end credits based on the amount of treasure Luigi has found. Completing the game once unlocks a second mode called the "Hidden Mansion", which features a stronger Poltergust and stronger ghosts. In the European and Australian version of this mode, the mansion appears as a mirrored reflection of the previous version, bosses are more difficult, ghosts and Portrait Ghosts are trickier to capture, and more ghosts appear in some of the rooms. Most of these Hidden Mansion differences were later added to the Nintendo 3DS remake of the game in all versions, although the stronger Poltergust and mirrored mansion were removed.

Luigi is notified about winning a mansion in a contest he never entered. He informs Mario and they agree to meet up there that evening. Luigi follows a map to the mansion, located in a dark forest, and finds it more sinister-looking than the supplied photo. With Mario nowhere to be found, Luigi enters the mansion alone. He encounters a ghost, which lunges at him, but is unexpectedly saved by a diminutive and elderly scientist who unsuccessfully tries to deter the ghost with a vacuum cleaner. They escape as more ghosts appear and the scientist introduces himself as Professor Elvin Gadd, or E. Gadd for short. He explains the mansion is supernatural in origin and only appeared a few days prior. E. Gadd tells Luigi that he saw Mario heading towards the mansion, but has not seen him since. Upon learning that Mario is Luigi's brother, E. Gadd entrusts Luigi with his ghost-hunting equipment, the Poltergust 3000 vacuum cleaner and Game Boy Horror communication device, as Luigi re-enters the mansion to find Mario.

As Luigi explores the mansion, he discovers that it was created by King Boo to shelter the now-freed special ghosts that E. Gadd had previously captured and turned into paintings with a large machine dubbed the "Ghost Portrificationizer"; King Boo subsequently sent the false notification of Luigi winning the mansion to lure the Mario Bros. into a trap in retaliation for the Boos they defeated in the past. Working his way through each of the darkened floors, halls, and locked rooms, Luigi recaptures the escaped ghosts and discovers that Mario has been trapped inside of a painting by King Boo and held captive in a secret ritualistic altar in the mansion's basement. While recapturing the last of the ghosts, Luigi finds the key that unlocks the altar's entrance and confronts King Boo, who, now disdainful over Luigi capturing his minions, pulls Luigi into the painting. Using his illusionary powers within a pocket dimension inside it, King Boo creates a suit-like replica of Bowser to combat Luigi. Using the replica's abilities against it, Luigi is able to force King Boo out of it and capture him. Returned to the real world in the aftermath, Luigi finds Mario unconscious but still trapped within the painting, which he triumphantly carries out of the mansion. Returning to E. Gadd's laboratory, he informs Luigi that the mansion has ceased to exist, due to it being an illusion, as they turn King Boo into a painting along with the last of the recaptured ghosts. Using the Ghost Portrificationizer's reverse function, Luigi frees Mario from his painting. In an epilogue, as a sign of gratitude towards Luigi for all his help, E. Gadd has a new non-haunted house for him built on the former site of the mansion, with its size determined by the amount of treasure collected during the game.