Kamen Rider Zi-O (仮面ライダージオウ, Kamen Raidā Jiō) is a Japanese tokusatsu drama produced by Toei Company and part of the Kamen Rider franchise. It is the twenty-ninth television series in the franchise and the twentieth and final installment of the Heisei period. The series premiered on September 2, 2018, following Kamen Rider Build's conclusion. It aired alongside Kaitou Sentai Lupinranger VS Keisatsu Sentai Patranger and later the miniseries Super Sentai Strongest Battle, before being joined by Kishiryu Sentai Ryusoulger in the Super Hero Time programming block.

The series incorporates a time travel and multiverse motif, serving as a tribute to the preceding Heisei-era Kamen Rider productions and functioning as a spiritual successor to Kamen Rider Decade. The series and its related films also feature returning actors reprising their roles from earlier Heisei entries.

Sougo Tokiwa, a high school senior born in 2000, dreams of becoming a king. He encounters Tsukuyomi, a mysterious girl from the year 2068, who warns him that he is destined to become Ohma Zi-O, the tyrannical "Overlord of Time." Despite his unease about this possible future, Sougo assumes the mantle of Kamen Rider Zi-O to protect the space-time continuum from the Time Jackers, a group attempting to alter the history of the Heisei-era Kamen Riders for their own purposes. He is assisted by Geiz Myokoin, a resistance fighter from 2068 who transforms into Kamen Rider Geiz and initially seeks to kill Sougo to prevent his rise to tyranny, and by Woz, a self-proclaimed prophet who supports Sougo's path toward becoming Ohma Zi-O and later becomes Kamen Rider Woz. Along their journey, Sougo and his allies encounter past Kamen Riders and their companions, earning their trust and acquiring their powers.

After graduating from high school, Sougo learns that Swartz, the leader of the Time Jackers, is Tsukuyomi's older brother and has been manipulating both Riders and his fellow Jackers in a plan to merge the entire Kamen Rider multiverse into a single world, which he intends to destroy in order to save his own. Sougo and his allies ultimately fail to prevent Swartz's scheme until Sougo embraces his destiny as Ohma Zi-O to defeat him. However, rather than ruling as foretold, Sougo sacrifices his powers to reverse the damage caused by Swartz, creating a new reality where he lives peacefully with Geiz, Tsukuyomi, and the remaining Time Jackers.

The trademark for Kamen Rider Zi-O was registered by Toei on April 12, 2018.

As the series pays tribute to the Heisei-era Kamen Rider productions, each tribute episode title includes a short phrase referencing a specific Heisei Kamen Rider series along with a year associated with it. The involvement of Zi-O's predecessor, Kamen Rider Decade, gradually becomes more prominent throughout the show. An asterisk (*) denotes episodes where his appearance, or that of his ally Kamen Rider Diend, coincides with another Rider's tribute episode.

The series is divided into four narrative arcs:

Kamen Rider Zi-O first appeared in a cameo in Kamen Rider Build the Movie: Be the One.