Nine Worlds

The characters of Kamen Rider Decade exist amongst various different iterations of reality within the series referred to as an A.R. World (Another Rider's  World). The Nine Worlds (9つの世界) are the A.R. Worlds that are based on the previous entries of the Kamen Rider Series that have aired during the Heisei period of Japanese history. Each differs in some way from the series on which it was based.

World of Kuuga
The World of Kuuga (クウガの世界) is the version of reality where Kamen Rider Kuuga takes place. It was the world where Yuusuke originated from before he joined Tsukasa in his journey. The backdrop used to arrive in this world shows a city with police cars and Mount Hitoki (灯溶山) in the background. Within this A.R. World, Tsukasa assumes the guise of a police officer.

World of Kiva
The World of Kiva (キバの世界) is the version of reality where Kamen Rider Kiva takes place. Unlike the original series, this world allows Fangires and humans to coexist, with the King's servants destroying the rebellious Fangires. The backdrop that allows entry into the World of Kiva depicts a nighttime cityscape with Castle Doran emerging from a building. Within this A.R. World, Tsukasa assumes the identity of a violinist.

World of Ryuki
The World of Ryuki (龍騎の世界) is the version of reality where Kamen Rider Ryuki takes place. In this world, there exists the Kamen Rider Trial (仮面ライダー裁判), a battle where the Kamen Riders fight each other within a parallel dimension known as the Mirror World (ミラーワールド) alongside their Contract Monsters (契約モンスター) in order for the victor to decide the fate of a defendant in a criminal case. Though no harm occurs to the Rider while in the Mirror World, a battle in the real world can be fatal and use of Vent cards outside a battle is forbidden. The backdrop used to arrive in this world shows Dragredder flying within the Mirror World as shown in a mirrored time display screen on a skyscraper. Within this A.R. World, Tsukasa assumes the identity of a lawyer.

World of Blade
The World of Blade (ブレイドの世界) is the version of reality where Kamen Rider Blade takes place. In this world, the Riders are high rank employees of the BOARD Corporation (株式会社BOARD), and are effectively "Kamen Rider Salarymen" (仮面ライダーサラリーマン), whose job it is to combat the Undead once receiving clearance to use Rouzers. In BOARD, there is a ranking system with Ace (エース) at the top followed by King, Queen, Jack, and the numbers with Two at the bottom. The backdrop used to arrive in this world shows a hand of facedown Rouse cards with the Ace of Spades (Blade's Change Beetle) face up. Within this A.R. World, Tsukasa assumes the role of chief waiter within BOARD. Also this is the world Daiki Kaitou first appeared.

World of Faiz
The World of Faiz (ファイズの世界) is the version of reality where Kamen Rider 555 takes place. In this world, students of the Smart Brain High School (スマートブレイン･ハイスクール) are being attacked nightly by monsters known as the Orphnoch. The backdrop used to enter the World of Faiz is a splitscreen featuring the Autovajin in Battle Mode and blue butterflies, referencing Smart Lady, a character from the original series who was Smart Brain Corporation's mascot of sorts. Within this A.R. World, Tsukasa assumes the role of a Smart Brain High student. Kaitou reveals his identity as Kamen Rider DiEnd in this world.

World of Agito
The World of Agito (アギトの世界) is the version of reality where Kamen Rider Agito takes place. Similar to the World of Kuuga, the Riders and the police battle the Gurongi. However, a new evil surfaces onto the world, monsters that they refer to as the Unknown. The backdrop used to enter the World of Agito is the elaborate tapestry seen in the opening sequence of Agito that depicts the story of Agito and the Unknown. This story arc further expands on the war between the Unknown and the Gurongi, which is hinted in both their respective series and explained more outside them. Within this A.R. World, Tsukasa assumes the role of a mailman with Toko's letter on his person.

World of Den-O
The World of Den-O (電王の世界) is the version of reality in which Kamen Rider Den-O takes place. Unlike the other Riders' the Worlds, the World of Den-O is identical to the setting of the original series. The only difference is the interior of the DenLiner having changed to that of the New DenLiner, among others. The backdrop used to enter this world depicts the DenLiner moving across the sands of time, which Ryutaros doodles on later in the episode arc. Within this A.R. World, Tsukasa finds himself wearing a trenchcoat and bucket hat, the same clothing as the Past Man. It should be noted that as Den-O's accurate reality is presented, connecting the TV series, the films, Decade itself, and other franchises, this can be referred to as the main continuity or the previous continuity prior to Kamen Rider Decade's A.R. Worlds.

World of Kabuto
The World of Kabuto (カブトの世界) is the version of reality where Kamen Rider Kabuto takes place. The backdrop used to enter this world depicts Tokyo Tower with an arm outstretched in homage to Souji Tendou's "walking the path to heaven" pose. Within this A.R. World, Tsukasa assumes the role of a ZECTrooper.

World of Hibiki
The World of Hibiki (響鬼の世界) is the version of reality where Kamen Rider Hibiki takes place. In this world, humans train to become Oni within one of three rival schools of sound-based martial arts called Ongekidō (音撃道), all based on different interpretations of the writings of the Grand Master (大師匠), the creator of Ongekidō. Tsukasa wears a pink kimono, what seems to be a reference to Street Fighter's Dan Hibiki. The masters and students of the Ongekidō styles wear traditional Japanese clothing, Tsukasa donning such attire while in the A.R. World. The backdrop used to enter this world depicts a forest with a taiko drum in the foreground. Nearly all of the original cast of Hibiki returns to reprise their roles, with only Shigeki Hosokawa (the original Hibiki) and Rakuto Tochihara (the original Asumu) absent. The episodes also show calligraphic representations of the Kamen Riders' names: "Tsukasa Kadoya" (門矢士), "Photo" (写), "passing through" (通), "Decade" (ディケイド), "Sea Cucumber" (海鼠), and Pink (桃色) for Decade; "Kuuga" (空我) for Kuuga; "Peach" (桃) for Den-O; and "Thief" (盗) and "Diend" (ディエンド) for Diend.

Novel
In the novel reimagination of the Kamen Rider Decade TV series, Kamen Rider Decade: The World of Tsukasa Kadoya ~The Garden Inside the Lens~, the Nine Worlds are not alternate realities, but in fact the original worlds. The Hikari Studio group has already traveled to 6 different worlds when the novel begins, leaving only Den-O, Kuuga and Kabuto’s world unexplored.