Abstract: An outdoor unit of an air-conditioning apparatus includes a housing, a heat exchanger, a bottom plate, a drainage structure, and a heating means. The housing has an opening through a side of the housing. The heat exchanger is disposed in an upper portion of the interior of the housing. The bottom plate is disposed at the bottom of the housing, and has a drainage hole through which to drain out drain water generated at the heat exchanger. The drainage structure is disposed below the heat exchanger and that guides the drain water to the bottom plate. The drainage structure includes a side panel that covers the opening of the housing, and a water guide plate disposed inside the housing and facing the side panel. The water guide plate and the side panel define a drainage path for draining the drain water between the water guide plate and the side panel.