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Super Smash Bros. Ultimate is a 2018 crossover fighting game developed by Bandai Namco Studios and Sora Ltd. and published by Nintendo for the Nintendo Switch. It is the fifth installment in the Super Smash Bros. series, succeeding Super Smash Bros. for Nintendo 3DS and Wii U. The game follows the series' traditional style of gameplay: controlling one of the various characters, players must use differing attacks to weaken their opponents and knock them out of an arena. It features a wide variety of game modes, including a campaign for single-player and multiplayer versus modes. Ultimate features 89 playable fighters, including all characters from previous Super Smash Bros. games alongside newcomers. The roster ranges from Nintendo mascots to characters from third-party franchises.

Planning for the game had begun by December 2015, with full development starting after the completion of 3DS/Wii U's downloadable content (DLC). Series creator and director Masahiro Sakurai returned along with Bandai Namco Studios and Sora, the studios that developed 3DS/Wii U, with the return of the studios speeding up the preparation process. Sakurai's goal with Ultimate was to include every character from previous games in the series despite the various development and licensing challenges this would present. Several well-known video game musicians contributed to the soundtrack, with Hideki Sakamoto writing the main theme "Lifelight".

Nintendo first teased Ultimate in a Nintendo Direct in March 2018 and fully revealed it at E3 2018 the following June. It later received two additional Directs before it was released on December 7, 2018. The game received universal acclaim, with some critics calling it the best of the series. They praised its amount of content and fine-tuning of existing Smash gameplay elements, although its online mode received criticism. Ultimate is the best-selling fighting game of all time, having nearly sold 25 million copies by June 2021. The game was supported with downloadable content adding new fighters, stages, and other content from its release until October 2021.