Provided herein is a sheet processing apparatus including a processing apparatus and a saddle unit. The saddle unit moves from a position inside the processing apparatus, along a direction orthogonal to a sheet transport direction, to a position outside the sheet processing apparatus. When the saddle unit is in the position outside the processing apparatus, a first transport guide is open to expose the outside of a first sheet transport path, and a second transport guide is open to expose the inside of the first sheet transport path. The saddle unit also includes a stapler, a folding blade and a pair of folding rollers, the first sheet transport path, and a second sheet transport path.