The sheet processing apparatus is provided with: a sheet aligning part that accumulates and aligns sheets so as to generate a bundle of sheets; an edge stitching processor that performs an edge stitching process by which the generated bundle of sheets is stitched together at one edge of the bundle of sheets; an apparatus housing that includes an opening portion through which the bundle of sheets is discharged to an outside of the apparatus housing; a saddle stitching processor that performs, outside of the apparatus housing, a saddle stitching process by which the discharged bundle of sheets is stitched together at a center portion of the bundle of sheets in a transporting direction of the bundle of sheets; and a stacking tray that is provided below the opening portion and outside the apparatus housing, and on which the bundles of sheets subjected to the edge stitching process and the saddle stitching process are capable of being stacked.