A sheet stacking apparatus includes a pair of sheet alignment mechanisms disposed to face each other in a direction of conveyance of a sheet or in a direction orthogonal to the direction of conveyance. Each one of the pair of sheet alignment mechanisms includes an aligning member, an elastic member, a driver, and a driving source. The aligning member reciprocates with reference to the sheet. The elastic member extends due to reaction force generated in response to pressing force applied to an edge of the sheet by the aligning member when a size of the sheet is larger than a prescribed size. The driver is coupled to the aligning member via the elastic member. The driving source transfers a driving force to the driver.