To provide a sheet post-processing apparatus configured to appropriately and successively stack stapled sheet bundles on a sheet discharge tray in the state where the stapled sheet bundles are arrayed while aligning the trailing ends thereof in the discharging direction with one another.

The sheet post-processing apparatus 20 includes: a binding device 23 configured to staple a plurality of sheets p after image formation by a stapling member 23a and form the sheet bundle P; a discharge device 24 to discharge the sheet bundles; and the sheet discharge tray 25 on which the sheet bundles discharged by the discharge device are stacked; wherein a guide member 27 for holding the sheet bundle discharged from the discharge device while projecting in the discharge direction of the sheet bundle is provided so as to be protruded and retracted between the discharge device and the sheet discharge tray.