A sheet processing apparatus and an image forming apparatus according to the present invention includes an upper blade capable of moving up and down and a lower blade to cut a sheet bundle between the upper blade and the lower blade by downward movement of the upper blade. When the sheet bundle is cut by the downward movement of the upper blade, an assisting sheet is moved to a position below a blade edge of the upper blade such that cutting waste of the cut sheet bundle is dropped down without being adhered to the lower blade.