Provided is a sheet post-processing device folding a sheet output from an image forming device at a predetermined target fold position, comprising: a first rotating body pair conveying the sheet; a second rotating body pair disposed along a sheet conveyance path and further downstream than the first rotating body pair in a sheet conveyance direction; a push unit moving a push member in a direction transverse to the sheet conveyance path so that, while the sheet is being conveyed, the push member comes into contact with the sheet and pushes the sheet into a nip of the second rotating body pair, the push member being disposed opposite the second rotating body pair across the sheet conveyance path; and a control unit controlling the push unit so that the pushing operation of the push member is performed at a timing preset according to the target fold position.