Masako Joh is a Japanese voice actress born on October 12, 1978, in Nagasaki Prefecture, Japan. She is affiliated with the talent agency 81 Produce. Her career in voice acting began in the late 1990s, with early roles in series such as Ojarumaru. She is recognized for her energetic performances and versatile vocal range, often bringing to life characters who are spirited, tomboyish, or quirky.

Joh rose to prominence through her role as Roll in all five seasons of the MegaMan NT Warrior anime series. This was a significant role that helped establish her in the industry. She is also well-known for voicing Ayumi Shorinji in the Inazuma Eleven franchise, a role she reprised in related video games and films. Her filmography includes a wide array of anime series. She played the school doctor Ryoko Mikado in To Love-Ru, the shrine priestess Mone in Gintama, and the duck Oliver in the NHK series Agatha Christie no Meitantei Poirot to Marple. Other notable credits include Kunba in Tengen Toppa Gurren Lagann, Megumi in Itazura na Kiss, and a role in Kobato. Her work extends beyond television anime to include video games, such as voicing Ingrid in Capcom Fighting Jam, and appearances in the mobile game Disney: Twisted Wonderland.

In addition to animation, Joh has contributed to various other media, including drama CDs, radio programs, digital comics, and narrating commercials. She has also provided voices for popular children's television segments in Japan. Her background includes stage performance, which has complemented her voice acting career.

Joh took a brief hiatus from her career starting in October 2012 for personal reasons but returned to voice acting in June 2013. She is known for her petite stature and is capable of speaking in the Nagasaki dialect.