To provide a sheet processing device that carries out additional sheet folding more appropriately than a sheet processing device with a conventional unit for additional sheet folding, and to provide a sheet folding method.

The sheet processing device includes: a stationary roller; a movable roller arranged to make contact with and separate from the stationary roller, and for folding a bundle of sheets in cooperation with the stationary roller by pressing the bundle of sheets; a heating part for heating one of the stationary roller and the movable roller.