To provide a sheet processing device that can perform highly accurate folding processing by preventing deviation of a folding position when performing folding processing.

The sheet processing device for processing a sheet on which an image is formed by an image forming apparatus includes: a conveying roller that conveys the sheet; a folding roller that bites the sheet to fold it; a folding plate that is driven in a perpendicular direction with respect to a conveying direction to make the folding roller bite the sheet being conveyed by the conveying roller; and a controller that controls sheet folding processing. The controller determines whether stiffness of the sheet is strong or weak based on attribute information of the sheet, and switches the control of the sheet folding processing based on the determined result.