Koga (written: 古賀), Kōga, Kouga or Kohga is a Japanese surname. Notable people with the surname include:

• Akira Koga (古賀 輝, born 1994), Japanese badminton player
• Aoi Koga (古賀 葵, born 1993), Japanese voice actress
• Atsushi Koga (古賀 淳, born 1993), Japanese former rugby union player
• Harue Koga (古賀 春江, 1895-1933), Japanese painter active in the Taishō period
• Hide Koga, nickname of Hidehiko Koga (古賀 英彦, 1899-1982), Japanese baseball player
• Hiroyasu Koga (古賀 浩靖, born 1947), Yukio Mishima's kaishakunin
• Issaku Koga (古賀 逸策, 1899-1982), often written as Issac Koga, Japanese inventor and scientist
• Issei Koga (古賀 一成, born 1947), Japanese politician from Democratic Party of Japan
• Junya Koga (古賀 淳也, born 1987), Japanese swimmer
• Kazunari Koga (古賀 一成, born 1972), Japanese former footballer
• Keitaro Koga (古賀 鯨太朗, born 1991), former Japanese footballer
• Koichiro Koga (古賀 幸一郎, born 1984), Japanese volleyball player
• Makoto Koga (古賀 誠, born 1940), Japanese Liberal Democratic Party politician
• Manabu Koga (古賀 学, 1935-74), Japanese swimmer
• Mary Koga (1920-2001), Japanese-American photographer and social worker in Chicago
• Masahiro Koga (古賀 正紘, born 1978), Japanese former footballer
• Masao Koga (古賀 政男, 1904-1978), was a Japanese composer, guitarist and pop musician
• Masato Koga (古賀 将之, born 1970), a former Japanese football player
• Masayuki Koga (古賀 正人), a Japanese shakuhachi player in USA
• Michiko Koga (古賀 美智子, born 1986) is the former stage name of F Chopper Koga (FチョッパーKOGA), Japanese bass player
• Mineichi Koga (古賀 峯一, 1885-1944), Fleet Admiral during World War II
• Minoru Koga (古賀 穂, born 1996), Japanese badminton player
• Mitsuki Koga (虎牙 光揮, born 1975), actor in martial arts films
• Nobuaki Koga (古賀 伸明, born 1952), Japanese unionist
• Sarina Koga (古賀 紗理那, born 1996), Japanese volleyball player
• Satoshi Koga (古賀 聡, born 1970), Japanese footballer
• Seiji Koga (古賀 誠史, born 1978), Japanese footballer
• Shuntaro Koga (古賀 俊太郎, born 1998), Japanese footballer
• Tadayoshi Koga (古賀忠義), pilot of the Akutan Zero in World War II
• Taichirō Koga (古賀 太一郎, born 1989), Japanese volleyball player
• Taiyo Koga (古賀 太陽, born 1998), Japanese football player
• Takahiro Koga (古賀 貴大, born 1999), Japanese football player
• Takeshi Koga (古賀 武, born 1939), retired Japanese judoka
• Takuma Koga (古賀 琢磨, born 1969), Japanese former footballer
• Takuma Koga (racing driver) (古賀 琢麻, born 19), Japanese stock car racing driver
• Toru Koga (古賀 徹, born 1967), a Japanese mixed martial artist
• Toshihiko Koga (古賀 稔彦, 1967-2021), a judo world champion
• Toshinori Koga (古賀 俊憲, born 1948), Japanese boxer
• Victor Koga (1935-2018), Japanese-Russian martial artist
• Yukiko Koga, anthropologist
• Yun Kōga (高河 ゆん, born 1965), Japanese manga artist
• Yūsei Koga (古賀 悠聖, born 2001), Japanese shogi player