A sheet stacker includes a sheet stacking portion, a guide portion, and an air blower. The sheet stacking portion is configured to stack a sheet bundle. The guide portion is configured to receive a leading end of a sheet conveyed toward the sheet stacking portion and guide the sheet downstream in a sheet conveyance direction in which the sheet is conveyed. The air blower is disposed above the sheet stacking portion and configured to blow air toward the sheet on a downstream side, in the sheet conveyance direction, from a position at which the guide portion receives the leading end of the sheet.