A creasing device forms a crease in a to-be-folded portion of a sheet. The creasing device includes a sheet-information reading unit that reads any one of sheet information and binding information; a determining unit that determines a surface, on which the crease is to be formed, of the sheet according to the one of the sheet information and the binding information read by the sheet-information reading unit; and a creasing unit that forms the crease on the surface determined by the determining unit.