A sheet stacking apparatus includes a body, a stacking portion which a sheet is stacked, a width regulating portion configured to move between a first position and a second position closer to a center of the stacking portion in a width direction orthogonal to a sheet feeding direction than the first position, and including a regulating surface configured to regulate a position, in the width direction, of the sheet stacked on the stacking portion, and a supporting portion pivotably supporting the stacking portion. The supporting portion is disposed on the body and closer to the center of the stacking portion in the width direction, than the regulating surface of the width regulating portion positioned at the first position.