The Weighted Companion Cube is an entity from the game Portal from Valve Software. In the game, the protagonist needs to activate switches to advance. Since the player can only be in one place at one time, Valve added the standard video-game cliché of 'crates' which can be manipulated in order to trip these switches or block areas. The designers thought it would be amusing to force the player to bring one of these crates along for the entire level - necessitating two-step-forward-one-step-back play and convoluted pathing in order to complete the task. During playtesting, however, they discovered that many players would 'forget' the crate, only to end up at the end without it. To encourage the player to bring the crate, they changed it to include pink hearts on each of its faces and inserted dialogue for GlaDOS, the AI antagonist. GlaDOS refers to the 'Weighted Companion Cube' as if it were actually a companion for the player, who spends the entirety of the game alone other than for the disembodied voice of GlaDOS. To increase emotional response, the player is eventually forced to incinerate the WCC in order to progress.

According to interviews with the design team, the idea for the WCC came about when Kim Swift, the Valve project lead, remembered reading articles on interrogation techniques which described how isolated people developed emotional attachments to inanimate objects. The team thought it funny to try to incorporate this in the game. Movies are rife with examples of this - 'Wilson' from the film Cast Away is a volleyball which serves as almost a second 'character' during Tom Hanks' long solo period.

Those who mourned its destruction can rejoice: The WCC is available as a plush toy!