A sheet folding apparatus including a folding roller, a positioning member, a push member, and a shift member. The folding roller including a pair of rollers to rotatably engage with each other to form a folding nip. The positioning member to support a first end of a printing medium which is fed along a folding path and to arrange the printing medium in an initial folding location. The push member to move to an insertion location to push the printing medium on the folding path into the folding nip, and to move to a retreat location. The shift member, located at an exit of the folding nip, to selectively guide the printing medium to a guide path to return the printing medium having passed through the folding nip to the folding path, the guide path being provided around at least one of the pair of rollers.