A sheet post-processing apparatus that is capable of aligning a sheet bundle accurately even if sheet types like sheet sizes are different, and is capable of setting a reference position depending on a selected finish. A conveyance unit conveys a sheet. A sheet stacking unit stacks sheets conveyed by the conveyance unit as a sheet bundle. A stapler applies a staple process to the sheet bundle stacked on the sheet stacking unit. A position determination unit determines a staple position at which the staple process is applied to the sheet bundle. An alignment unit aligns the sheets stacked on the sheet stacking unit in a direction perpendicular to a sheet conveyance direction with respect to one of both sides parallel to the sheet conveyance direction. The alignment unit determines the one side as a reference side depending on the staple position determined by the position determination unit.