The invention relates to a sheet processing apparatus, an image forming system, and a method of enhancing folding of a sheet bundle. The sheet processing apparatus includes: a folding unit that folds a sheet bundle; a pressing unit that presses a fold part of the sheet bundle, which has been folded by the folding unit, in a thickness direction of the sheet bundle to enhance folding of the fold part; and a moving unit that advances and returns the pressing unit in a width direction of the sheet bundle. The pressing unit starts pressing from a predetermined position in the width direction of the sheet bundle during advancing movement, and releases pressing after going past one end of the sheet bundle. The pressing unit starts pressing from the predetermined position during return movement and goes past the other end of the sheet bundle.