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 Endo (遠藤 誠一, 1960–2018), Japanese cult member
• 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 (巻田 清一, born 1968), Japanese footballer
• Seiichi Miyake (三宅 精一, 1926–1982), Japanese inventor
• Seiichi Morimura (森村 誠一, 1933–2023), 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 Osanai (長内 清一, born 1953), Japanese sport wrestler
• Seiichi Ota (太田 誠一, born 1945), Japanese politician
• Seiichi Saito (斉藤 誠一, born 1976), Japanese footballer
• Seiichi Sakiya (崎谷 誠一, born 1950), Japanese footballer
• Seiichi Shima (嶋清 誠一, 1920-1945), Japanese baseball player and soldier
• Seiichi Sugano (菅野 誠一, 1939–2010), Japanese aikidoka
• Seiichi Suzuki (philologist) (鈴木 誠一, born 1956), Japanese philologist
• Seiichi Suzuki (figure skater) (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 (田中 誠一, born 1943), 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