Seiichirō Yamashita is a Japanese voice actor born on May 21, 1992, in Fukuyama, Hiroshima Prefecture. He is affiliated with the talent agency Office Osawa.

Initially interested in becoming an actor, Yamashita found it challenging to pursue this path from his hometown. During high school, his deep engagement with anime and manga, shared with friends, inspired him to aim for a career in voice acting. After graduating from high school, he moved to Tokyo and enrolled in the Amusement Media Academy, a vocational school for entertainment. He made his professional debut in 2012 with a role in the Japanese dub of the live-action series The Good Doctor while still a student. His first leading role in a television anime came in 2013 as Takahiro Mizuno in Walkure Romanze.

Yamashita's career is marked by a wide range of roles across various genres, demonstrating his versatility. Early in his career, he gained recognition for roles such as Kakeru Naruse in the romantic drama Orange and Rekka Yukimura in Battle Spirits: Burning Soul. He has also been a consistent presence in the Touken Ranbu franchise, voicing the characters Yagen Tōshirō and Aizen Kunitoshi across multiple anime adaptations and video games.

He has taken on prominent roles in several popular series of the early 2020s. He voiced Toru Ishikawa in the romantic comedy Horimiya and its sequel, Horimiya: The Missing Pieces. In the mecha anime 86, he played the role of Raiden Shuga, a supporting yet integral member of the main squadron. His performance as the protagonist Cid Kagenou, also known as Shadow, in the isekai series The Eminence in Shadow earned him the Best Voice Acting Performance award at the 10th Anime Trending Awards in 2024. Other notable lead roles include Jirō Yakuin in More Than a Married Couple, But Not Lovers and Sōichirō Tachibana in Aoashi.

Beyond anime, Yamashita is also active in video games, with roles including Ace Trappola in the popular mobile game Twisted Wonderland and multiple characters in the Touken Ranbu game series. He has also performed theme songs for projects related to franchises like Twisted Wonderland and Touken Ranbu. His filmography includes roles in theatrical films such as Black Butler: Book of the Atlantic and Orange: Future. Yamashita continues to be an active and prominent figure in the voice acting industry.