A sheet processing apparatus includes an overlap conveying unit that conveys two or more sheets in an overlapping manner; a stacking unit that stacks therein the sheets conveyed in the overlapping manner; an aligning unit that includes a rotating body and a regulating member and is configured to bring the rotating body into contact with a topmost sheet among the stacked sheets, so as to make the sheet abut the regulating member and align ends of the sheets; and a controller that controls the overlap conveying unit. The controller is configured to determine, according to sheet information, whether to make the topmost sheet overlap in a condition of projecting from the other sheets.