Mayumi Yamaguchi is a Japanese voice actress, or seiyū, born on May 12, 1975, in Iwate Prefecture, Japan. She is affiliated with the talent agency Accent. Her career in voice acting began in 1999 and she is known for her dynamic and versatile performances, often portraying powerful, bold, or androgynous characters.

Yamaguchi's debut role was Gabumon in the long-running Digimon Adventure television series in 1999. This marked the start of a lasting association with the Digimon franchise. She has consistently voiced the character Gabumon, along with its various evolutionary forms like Garurumon and MetalGarurumon, across numerous productions. Her filmography includes the original Digimon Adventure, Digimon Adventure 02, and the recent film Digimon Adventure: Last Evolution Kizuna. She also reprised the role in the Digimon Adventure tri. film series, including the final installment Digimon Adventure tri.: Future. Beyond the original Adventure continuity, she voiced a different major character, Henry Wong, in the third television series, Digimon Tamers.

In addition to her extensive work in the Digimon universe, Yamaguchi is well-recognized for several other prominent roles. She voiced the homunculus Envy in the 2003 Fullmetal Alchemist television series and its related original video animations, such as Fullmetal Alchemist: Chibi Party and Fullmetal Alchemist: Premium Collection. She also played Forte Stollen in the Galaxy Angel anime series, including Galaxy Angel A, Galaxy Angel S, and Galaxy Angel Z. Her other notable television roles include Bamū in Zatch Bell!, Tarō Myōjin in The Law of Ueki, Sousuke Kawara in Samurai Champloo, Orochimaru as a child in Naruto, Ganta in Kero Kero Keroppi: Hasunoue Town Kikiippatsu!, Hagan in Tanken Driland - 1000-nen no Mahō -, and the Chinese third brother in Jinzo Konchu Kabuto Borg VxV.

Yamaguchi's voice work also extends to original video animations and video games. In the OVA series The Immorals, she is credited for the role of Sumire Arimiya under the name Moko Hanamaki. Her video game credits include the role of the male Pokémon Trainer in the Super Smash Bros. series, as well as reprising her anime roles of Forte Stollen in the Galaxy Angel and Galaxy Angel II games, and Gabumon in various Digimon titles.

Before launching her voice acting career, Yamaguchi graduated from the Voice Acting Department of Yoyogi Animation Academy. Following her education, she gained experience as a member of the Free Atom theater troupe and later co-founded the P-9 Contrapunt theater group. As of 2024, she continues to be active in the industry, taking on new roles in series such as Onipan! and reprising classic ones in projects like Digimon Adventure -Beyond-.