Kōsaku, Kosaku or Kousaku (written: 功作, 宏作, 幸作, 耕作, 耕筰, 興作, 孝作 or 浩作) is a masculine Japanese given name. Notable people with the name include:
Kosaku Akimoto (秋元 宏作) (born 1968), Japanese baseball player
Kōsaku Aruga (有賀 幸作) (1897–1945), Imperial Japanese Navy admiral
Kōsaku Hamada (濱田 耕作) (1881–1938), Japanese archaeologist and academic
Kosaku Masuda (増田 功作) (born 1976), Japanese footballer
Kōsaku Matsumora (松茂良 興作) (1829–1898), Japanese karateka
Satoyama Kosaku (里山 浩作) (born 1981), Japanese sumo wrestler
Kosaku Shimada (born 1944), Japanese golfer
Kōsaku Takii (滝井 孝作) (1894–1984), Japanese poet and writer
Kosaku Yamada (山田 耕筰) (1886–1965), Japanese composer and conductor
Yoshio Kōsaku (吉雄 幸作) (1724–1800), Japanese physician and translator
Kōsaku Yosida (吉田 耕作) (1909–1990), Japanese mathematician
