A sheet post-processing device for post-processing of a recording sheet ejected from an image forming device in an ejection direction, the sheet post-processing device including: a first tray on which the recording sheet is placed after ejection; a push-out member that pushes out, in a push-out direction different from the ejection direction, the recording sheet placed on the first tray; and a guide member adjacent to an ejection direction downstream end of the first tray, the guide member supporting a portion of the recording sheet that protrudes from the first tray in the ejection direction. The guide member includes a resistive force reduction unit that reduces a resistive force that occurs due to contact between the guide member and the recording sheet, the resistive force hindering the pushing out of the recording sheet when the push-out member pushes out the recording sheet.