A sheet stacking apparatus includes an alignment unit configured to align a sheet stacked on a stacking tray in a width direction which is orthogonal to a direction in which the sheet is discharged. The alignment unit includes first and second alignment members configured to move in the width direction. The first and second alignment members come into contact with side ends in the width direction of the sheet stacked on the stacking tray to align the sheet. When a second sheet of a different length in the width direction from that of a first sheet is stacked while shifted in the width direction on the first sheet which is already stacked on the stacking tray, the sheet stacking apparatus prohibits an alignment operation of the alignment unit on the second sheet.