A sheet processing apparatus includes a holding portion that holds a booklet, which includes folded sheets, by a pair of holding members, a pressing portion having a first pressing member, which is brought into contact with a spine of a booklet held by the holding portion so as to perform a deforming process to the spine, and a second pressing member having a thickness, in the thickness direction of the booklet, greater than that of the first pressing member, a moving portion that moves the pressing member along the spine, and a controlling portion that changes the first pressing member to the second pressing member, and increases a deformation amount from the spine before the deformation to a pressing position in the pressing direction where the second pressing member presses the spine, when the thickness of the booklet held by the holding portion exceeds a predetermined thickness.