|This article is about a/an transformation device in Kamen Rider Fourze and Kamen Rider OOO.|
- "Same, Kujira, Ookamiuo!"
- ―Transformation announcement[src]
The Poseidon Driver (ポセイドンドライバー Poseidon Doraibā) was a transformation belt created by the Kougami Foundation to use the power of a trio of Core Medals (Same, Kujira, and Ookamiuo) they had created in the future, used by Michal Minato to transform into Kamen Rider Poseidon, a form which would be taken by the Greeed which had formed within him.
Like the OOO Driver, the Poseidon Driver also utilizes Core Medals and is somewhat simpler-looking, shaped in a circle with outer design. Unlike the way OOO arranges his Core Medals to transform, Poseidon had his Core Medals put in an inverse triangular position on the Driver rather than being straight and diagonal like OOO's. It does not require a scanner like OOO does.
The Poseidon Driver was only shown to utilize the Same, Kujira, and Ookamiuo Medals. It is unknown it was compatible only with that particular set or could use the other Core Medals to Mix & Match like OOO.
About forty years after Eiji Hino first transformed into the modern Kamen Rider OOO, at a time when new monsters had appeared to threaten humanity, the Poseidon Driver was created along with a set of three new Core Medals by the Kougami Foundation to give birth to a new Rider that could use the Medal System. The elderly Kousei Kougami gave the Driver to Michal Minato, who couldn't use his Aqua Driver due to his insecurities as well as his fear of water. Kougami believed that the Poseidon Driver could help Michal overcome his timidness and fear, and grant him more power. However this proved futile, and even as Poseidon, Michal couldn't win against the creatures of his time, lost, and Michal was knocked out of transformation. However, just as he was about to give up, the black hole that was created by the death of Kiyoto Maki forty years earlier appeared, and the Core Medals and Cell Medals from the past entered his body, resulting in a Greeed forming inside the Poseidon Medals. This new Greeed developed an intense desire to fight, and Michal was almost instantly overwhelmed by the new being. With Michal's personality subdued, Kamen Rider Poseidon fought in their home time period for a brief while, but after a time anomaly formed as a result of a meteor containing SOLU crashing into the ocean in the present day, the Poseidon entity saw its chance and dived through, heading off to fight the Kamen Riders of the past.
Arriving forty years in the past, the Greeed Kamen Rider easily overwhelmed a small troop of the Kougami Foundation, but is quickly intercepted by Ankh and Eiji Hino, who took some of his Cores in order to fight back, with Poseidon fleeing to recuperate. Without the extra Cores to stabilize him, Michal started to regain control. Eiji confronted the dark Rider, and suppressed the Greeed's personality, but it soon resurfaced. After Kamen Rider Poseidon lost more of the Core Medals in his body to Eiji and Ankh, the Greeed seperated from his human host, gaining his own body in the form of Kamen Rider Poseidon. Gaining his courage thanks to Eiji's words of being a Kamen Rider, Michal returned as Kamen Rider Aqua and helped OOO defeat Poseidon. With Poseidon destroyed, the Driver was lost with him, while the future Core Medals were claimed by Foundation X as part of Lem Kannagi's ambition to become the Super GingaOh.