A sheet processing apparatus and a method of controlling the same align sheets stacked on a stacking unit, by causing a first alignment member and a second alignment member to come into contact with edges of a sheet stacked on the stacking unit in a sheet width direction. In a case that a second sheet that is different from a first sheet stacked on the stacking unit is to be stacked on the first sheet and aligned using the first alignment member and the second alignment member, control is performed to discharge a partition sheet onto the first sheet stacked on the stacking unit.