An outdoor unit of an air-conditioning apparatus includes a housing forming an outline of the outdoor unit, a partition plate extending from a front surface toward a rear surface of the housing and partitioning an interior of the housing into an air-sending device chamber and a machine chamber, a heat exchanger disposed in the air-sending device chamber and along an end portion located at the rear surface of the housing, and a holder mounted on the heat exchanger and accommodating a temperature-measurement unit that measures an outside air temperature. The partition plate includes a first flat portion disposed with a space between a rear surface of the first flat portion and the heat exchanger and extending in a longitudinal direction of the heat exchanger, and a second flat portion projecting from a side edge portion of the first flat portion toward the heat exchanger.