A robot cleaner and a control method thereof may judge whether or not water is received in the robot cleaner performing wet cleaning. The robot cleaner includes a main body, moving units, a cleaning unit mounted on the main body and contacting a floor surface to perform cleaning, a water supply unit supplying water to the cleaning unit, and a sensing unit provided on at least a portion of the water supply unit to sense whether or not there is water within the water supply unit. The sensing unit includes a housing, a transmission part radiating electromagnetic waves, a reception part receiving the electromagnetic waves radiated by the transmission part, and a stepped part provided on at least a portion of the housing along a moving path of the electromagnetic waves radiated by the transmission part and received by the reception part.