A cushion body for sitting is provided which can offer good feeling of sitting even to many unspecified users of various body shapes and allow them to be seated more comfortably. A cushion body for sitting (1) includes: a base part (2) having a sitting surface (S1) formed of a curved surface that is curved concavely to support, from below, the buttocks of a seated person in a seated posture; and a pair of side supporting portions (3) each having a side supporting surface (S2) formed of a curved surface that is curved continuously from the sitting surface (S1) of the base part (2), where the pair of side supporting portions (3) project from sides of the base part (2) such that the side supporting portions (3) can be bent and deformed by a load applied to the side supporting surfaces (S2), so as to support both the left and right parts of the buttocks of the seated person in the seated posture.