A sheet processing apparatus includes: a staple tray on which sheets are stacked temporarily and stapled after being aligned together and from which the stapled sheets are discharged to a discharge tray; a discharger to discharge the sheets to the staple tray; a rear end reference against which rear ends of the sheets are abutted to be aligned together; a returner to return the sheets discharged by the discharger and to align the sheets together in a conveying direction based on the rear end reference fence; a lifter to lift and lower the returner with respect to a sheet; and a guide configured to guide the sheets toward the rear end reference fence when the sheets are returned and to be lifted and lowered in synchronization with the lifting and lowering by the lifter.