A sheet stacking apparatus includes a sheet stacking unit having a stacking surface on which a sheet is stacked, a sheet regulating unit, and an opposed member. The sheet regulating unit includes a main body having a regulating surface which regulates a position of the sheet and being movable with respect to the sheet stacking unit, and an operating portion movably supported on a top of the main body and having a pressure surface. The operating portion is movable from a first position to a second position by performing a pressing operation on the pressure surface. The main body is prevented from moving when the operating portion is in the first position, and is allowed to move when the operating portion is in the second position. The opposed member is removably supported on the main body and opposed to at least a part of the pressure surface and an upper surface of the operating portion.