A sheet processing apparatus including: a saddle stitching stapler including a staple driving portion movable in a sheet width direction and a staple clinching portion arranged at a saddle stitching position, and adapted to staple a middle portion of a sheet bundle by causing the staple driving portion to approach the staple clinching portion after the staple driving portion moves to the saddle stitching position; and a side stitching stapler including a staple driving portion and a staple clinching portion pivoted to each other, and adapted to staple an end portion of the sheet bundle by holding the end portion between the staple driving portion and the staple clinching portion after the staple driving portion and the staple clinching portion move to a side stitching position in the sheet width direction, the side stitching stapler and the saddle stitching stapler being aligned in the sheet width direction.