The recording medium post-processing apparatus is provided with: a recording-medium collecting member that collects plural recording mediums transported from outside as a recording-medium bundle; a stitching unit that stitches the recording mediums having: a supporting portion which supports the recording-medium bundle transported from the recording-medium collecting member; and a stapler portion which pushes a stitching needle into the supported recording-medium bundle; and a recording-medium bundle stacking member that is arranged below the stitching unit and stacks the recording-medium bundle stitched by the stitching unit in a vertical direction. The supporting portion and the stapler portion of the stitching unit are moved outside a position at one edge portion of the recording-medium bundle when the recording-medium bundle is not stitched.