A cushion for a chair includes a cushion body (20) having a mediate position provided with a convex waist support portion (21), an upper end provided with an inclined backrest support portion (22) and a lower end provided with a concave resting portion (24). Thus, the backrest support portion and the waist support portion of the cushion body are designed to fit the user's back and waist ergonomically so that the cushion body can support the user's back and waist smoothly and exactly. In addition, the upper buffering space (202) and the lower buffering space (201) of the cushion body provide a buffering effect to allow deformation of the cushion body so as to regulate the user's seating gesture and to provide a comfortable sensation to the user.