A sheet sorting apparatus includes a second tray arranged below a first tray detachable from a main body having a first discharge port corresponding to the first tray and a second discharge port corresponding to the second tray, a discharge unit, a tray detecting unit, and a control unit. The discharge unit discharges a sheet toward the first or second discharge port. In a state that the first tray is detached from the main body and the discharge unit discharges sheets from the first discharge port to the second tray and then the first tray is detected as attached to the main body, the control unit selects a discharge destination of a second sheet, subsequent to a first sheet discharged and stacked first after the first tray is attached to the main body, such that the second sheet is stacked on a tray having the first sheet thereon.