Tadahisa Saizen is a Japanese voice actor born on September 25, 1964, in Osaka City, Osaka Prefecture. He is a graduate of Kyoto Seika University and is affiliated with the talent management firm ARTSVISION. Prior to his voice acting career, which began in 1992, Saizen worked as a high school teacher. His early career also involved work with Kansai Geijutsuza and the Theater Company Zenshinza.

Saizen is perhaps best known for his long-standing association with the Fate franchise, a series of visual novels, anime, and games. Within this universe, he provides the voice for the character Berserker, whose true identity is the Greek hero Heracles. He has voiced this role in numerous adaptations, including the anime series Carnival Phantasm and various video games. He also voices the pirate Edward Teach, also known as Blackbeard, in the franchise, with credits including the OVA series Fate/Grand Carnival and the game Fate/Grand Order.

Beyond his prominent roles in the Fate series, Saizen has built a extensive career with a wide range of character voices. He voiced the character Shiro in multiple entries of the UFO Ultramaiden Valkyrie series, including UFO Ultramaiden Valkyrie 2: December Nocturne and UFO Ultramaiden Valkyrie 4: Banquet of Time, Dreams, and Galaxies. In the anime adaptation of the visual novel Yamibo - Darkness, the Hat, and the Travelers of the Books, he took on the role of Gorilla. His work in the early 2000s also includes the role of Kintoki Miyazawa in the series Shootfighter Tekken. Saizen's filmography includes numerous other credits across popular anime, such as roles in Inuyasha, Blood+, Samurai 7, and Firefighter! Daigo of Fire Company M.