Koichi Yamadera (山寺 宏一 Yamadera Kōichi) is a Japanese voice actor that affiliated to Across Entertainment.
- Ultraman G - Voice dub of Lloyd Wilder
- Gridman the Hyper Agent: boys invent great hero - Voice of Takeshi Todo
- The・Ultraman: Jackal vs. Ultraman: Voices of Demon King Jackal, Zoffy, Melos and other male characters
- Ultraman Orb The Movie: Let Me Borrow the Power of Bonds! - Voice of Alien Gapiya Sadis
- Ninja Scroll - Voice of Jūbei Kibagami
- Cowboy Bebop - Voice of Spike Spiegel
- Dragon Ball Z: Battle of Gods - Voice of Beerus: The God of Destruction
- Ghost in the Shell - Voice of Togusa
- Mashin Eiyuuden Wataru - Voice of Kurama Wataribe
- Neon Genesis Evangelion - Voice of Ryoji Kaji
- Anpanman - Voice of Cheese
- Ranma ½ - Voice of Ryōga Hibiki
- Lupin III - Voice of Koichi Zenigata (replacing the late Goro Naya)
- One Piece - Voice of Donquixote Rosinante
- GinTama - Voices of Yoshida Shouyou and Utsuro
- Hunter x Hunter (2011 version) - Voice of Silva Zoldyck
- Samurai Pizza Cats - Voice of Bad Bird
- Sword Art Online - Voice of Akihiko Kayaba
- Danganronpa V3: Killing Harmony - Voices of Monokuma Kubs
- Mario Kart Arcade GP 2 - Voice of Color Commentator (JP)
- Batman Ninja - Voice of Bruce Wayne/Batman
- Super Robot Wars - Voice of Daisuke Umon/Duke Fleed (Replacing the late Kei Tomiyama)
- Koichi Yamadera's favorite Ultra Warriors are Ultraman, Ultraseven and Ultraman Ace.
- Discounting the Jackal vs Ultraman, Sadis in Orb the Movie is his first role in the Ultraman Series since Lloyd Wilder in Ultraman G. Because of this, he was excited to return to the franchise and casted as Sadis. Although initially nervous about this role, the director encouraged ad-libs in his role.
- He's also known for his versatile range, as protagonists, antagonists, and even to the point of voicing as a female.