Paper Mario: The Thousand-Year Door is a 2004 role-playing video game developed by Intelligent Systems and published by Nintendo for the GameCube. The Thousand-Year Door is the second game in the Paper Mario series.

The Thousand-Year Door borrows many gameplay elements from its predecessor, such as a drawing-based art style, and a turn-based battle system with an emphasis on action. For the majority of the game the player controls Mario, although Bowser and Princess Peach are playable at certain points. The plot follows Mario's quest as he tries to retrieve the seven Crystal Stars and rescue Peach from the X-Nauts.

The game was praised by critics. Critics generally praised the game's engaging plot and gameplay. The Thousand-Year Door won the "Role Playing Game of the Year" award at the 2005 Interactive Achievement Awards.