A sheet folding apparatus includes: a saddle-stitching unit configured to stitch a center of a sheet bundle; a folding unit configured to fold the sheet bundle at the center to form a fold; a loading base onto which the sheet bundle conveyed from the folding unit is loaded; a nipping plate configured to be pressed to and separated from the loading base in parallel to the loading base and to nip the sheet bundle loaded onto the loading base; and first and second rollers that move along a direction of the fold while nipping and pressing the fold of the sheet bundle nipped by the nipping plate to reinforce the fold. Here, a surface, which faces the loading base, of the nipping plate is provided with an elastic member.