Koichi Yamadera (山寺宏一, Yamadera Kōichi, born June 17, 1961) is a Japanese actor, narrator and singer from Shiogama, Miyagi Prefecture. He graduated from Tohoku Gakuin University's economics school and is currently affiliated with Across Entertainment. Before that, he was affiliated with the Tokyo Actor's Consumer's Cooperative Society.

He is known for his roles in Cowboy Bebop (as Spike Spiegel), the Yakuza videogame series (as Shun Akiyama), Shōwa Genroku Rakugo Shinjū (as Sukeroku), Ghost in the Shell (as Togusa), Neon Genesis Evangelion (as Ryoji Kaji), Gintama (as Shoyo Yoshida / Utsuro), Ninja Scroll (as Jūbei Kibagami), Dragon Ball Super (as Beerus), Anpanman (as Cheese), Ranma ½ (as Ryōga Hibiki/P-chan and the Jusenkyō Guide), Space Battleship Yamato 2199 (as Aberdt Desler), Pretty Cure All Stars DX 3 (as Black Hole) and Lupin III (as the official voice of Koichi Zenigata since 2011). He's been in every Pokémon film to date, though always in different roles, the exception being movie 22, in which he reprised his role as Mew from the first movie, of which movie 22 is a remake. He also voiced Mew in some collateral projects. For Disney, he is the official dubbed voice of Donald Duck, Genie and Stitch (among others) in Japanese.

In the live action field, he is the official dub-over artist of Will Smith (Yamadera met Smith several times), Eddie Murphy and Jim Carrey. He is also known for voicing Chris Tucker, Jean-Claude Van Damme, Robin Williams, Mike Myers, Stephen Chow, Brad Pitt, Charlie Sheen, Michael Keaton, Michael J. Fox, Will Ferrell, Chris Pratt and Jon Hamm in the Japanese language releases of their respective films.