Koji Seki, born November 25, 1982, is a Japanese voice actor from Saitama Prefecture. He stands 169 cm tall. Seki is affiliated with the talent agency Raccoon Dog. He graduated from the Pro-Fit Voice Actor Training School and originally joined the agency Pro-Fit before transferring to Raccoon Dog on April 1, 2022.

Seki has built a career centered on supporting and guest roles across a wide range of anime series. His notable anime appearances include Rahden in SPY×FAMILY, Kai Akakura in Haikyuu!! To the Top, Naijan in Sword Art Online: Progressive - Scherzo of Deep Night, Greg Zangief in Let This Grieving Soul Retire, and various goblins and officials in the Sword Art Online franchise. Other roles include Chikaaki in The Elusive Samurai, Sebas in The Aristocrat's Otherworldly Adventure, a detective in Mission: Yozakura Family, and Tsuzumi in Boruto: Naruto Next Generations. In video games, he has voiced characters such as Cormag in Fire Emblem Heroes and Hokma in Library of Ruina. Beyond his work in Japanese productions, he also provided the voice of Eno's Father in the Star Wars: Visions episode of the same name.

Outside of his voice work, Seki has listed his hobbies as touring ramen shops, touring liquor shops, lure fishing, and playing games. He has also demonstrated a special skill in impersonating the character Nagasawa from the series Chibi Maruko-chan.