There is provided a sheet post-processing device that has a first tray, outlet rollers, and a control section. The first tray holds sheets for post-processing with respect to the sheets supplied from an image forming device. The outlet rollers discharge the sheet to the first tray. The control section controls a rotational speed of the outlet rollers to be decelerated after a leading end portion of the sheet passes through the outlet rollers and before the leading end portion of the sheet comes into contact with the first tray.