Kōzō Sasaki (佐々木 更三, Sasaki Kōzō, 25 May 1900 – 24 December 1985) was a Japanese socialist agitator and politician. The longtime leader of the left wing of the Japan Socialist Party in the postwar period, he served as chairman of party from 1965 to 1967. Small in stature and possessed of a fiery temper and a thick rural Tōhoku accent, he was a loud-mouthed and charismatic figure nicknamed "Kōzō the Ogre."