Seiichi (written: 誠一, 聖一, 清一, 精一, 整一, 成一, 勢一, 盛一 or 晟一) is a masculine Japanese given name. Notable people with the name include:
Seiichi Akamine (赤嶺 誠一, 1920–1995), Japanese karateka
Seiichi Eto (衛藤 晟一, born 1947), Japanese politician
Seiichi Furuya (古屋 誠一, born 1950), Japanese photographer
Seiichi Hatano (波多野 精一, 1877–1950), Japanese philosopher
Seiichi Hishikawa (菱川 勢一, born 1969), Japanese art director
Seiichi Iju (伊集 盛一), Japanese karateka
Seiichi Ishii (石井 精一, born 1967), Japanese video game designer
Seiichi Itō (伊藤 整一, 1890–1945), Imperial Japanese Navy admiral
Seiichi Iwao (岩生 成一, 1900–1988), Japanese academic, historian and writer
Seiichi Kanai (金井 清一, born 1940), Japanese golfer
Seiichi Kaneta (金田 誠一, born 1947), Japanese politician
Seiichi Kuno (久納 誠一, 1887–1962), Japanese general
Seiichi Makita (born 1968), Japanese footballer
Seiichi Morimura (森村 誠一, born 1933), Japanese writer
Seiichi Motohashi (本橋 成一, born 1940), Japanese photographer
Seiichi Negishi (根岸 誠一, born 1969), Japanese footballer
Seiichi Niikuni (新国 誠一, 1925–1977), Japanese poet and painter
Seiichi Ogawa (小川 誠一, born 1970), Japanese footballer
Seiichi Ōmura (大村 清一, 1892–1968), Japanese politician
Seiichi Ota (太田 誠一, born 1945), Japanese politician
Seiichi Sakiya (崎谷 誠一, born 1950), Japanese footballer
Seiichi Sugano (菅野 誠一, 1939–2010), Japanese aikidoka
Seiichi Suzuki (鈴木 誠一, born 1976), Japanese figure skater
Seiichi Tagawa (田川 誠一, 1918–2009), Japanese politician
Seiichi Takamura (高村 誠一, born 1960), Japanese handball player
Takebayashi Seiichi (武林 盛一, 1842–1908), Japanese photographer
Seiichi Tanabe (田辺 誠一, born 1969), Japanese actor
Seiichi Tanaka, American musician
Seiichi Tejima (手島 精一, 1850–1918), Japanese educator
Seiichi Tobata (東畑 精一, 1899–1983), Japanese scientist
Seiichi Uchikawa (内川 聖一, born 1982), Japanese baseball player
Seiichi Yamamoto (山本 精一, born 1958), Japanese musician
