A sheet post-processing device includes a folding portion, a discharge portion, a stacking tray, a shift unit, a pair of alignment members, and a controller. The pair of alignment members include first and second alignment members at one and the other side of sheets stacked on the stacking tray. When a sheet bundle sorted and stacked on the stacking tray includes a folded sheet, executes a second alignment mode in which it performs, for sheets shifted to a first position at one side on the stacking tray, alignment with only the first alignment member and, for sheets shifted to a second position at the other side, alignment with only the second alignment member, or does not perform alignment process.