The post-processing device includes: a binding unit that forms a cut in a sheet stack and cuts a part of the sheet stack into a predetermined shape to form a tongue portion in the sheet stack, the tongue portion having a part where one end part of the tongue portion is not separated from the sheet stack, and binds the sheet stack by bending the tongue portion and inserting the other end part of the tongue portion into the cut; and a sheet stack transport unit that transports the sheet stack in an orientation such that the one end part of the tongue portion in the sheet stack bound by the binding unit is on a downstream side of the other end part of the tongue portion in the sheet stack transport direction.