A sheet processing apparatus includes a pair of clamp units, a first pressing unit and a second pressing unit. The first pressing unit performs a square back process on a spine of the sheet bundle by moving the first pressing unit in a width direction of the sheet bundle in a state in which the spine of the sheet bundle is pressed toward the pair of clamp units. The second pressing unit performs a pressing process on a folding line of the sheet bundle by moving in the width direction of the sheet bundle while nipping the folding line of the sheet bundle.