A sheet folding device includes a first folding unit, a retreat path, a sheet retaining mechanism, and a second folding unit. The first folding unit forms a first fold in a sheet. The retreat path is entered by the sheet having the first fold formed by the first folding unit, and has a stop portion to be contacted by the first fold. The sheet retaining mechanism moves a movable member with respect to the sheet that enters the retreat path, and regulates movement of the sheet in the retreat path in a direction of thickness of the sheet. The second folding unit forms a second fold in the sheet having the first fold while allowing the sheet that has contacted the stop portion to exit the retreat path.