A sheet stacker includes a sheet stacking member, a biasing force applier, an angle changer, and a biasing force change reducer. The sheet stacking member has an upstream portion in a sheet conveyance direction. The upstream portion is movable in a vertical direction. The biasing force applier is configured to bias the sheet stacking member upward. The angle changer is configured to change an angle of a sheet stacking face of the sheet stacking member, relative to a surface of a sheet conveyed through a sheet conveying portion. The biasing force change reducer is configured to restrain a change of a biasing force acting on the sheet stacking member, before and after a change of the angle of the sheet stacking member by the angle changer.