A robot cleaner includes a main body forming an exterior and having a water tank housing formed at a rear side thereof. The water tank housing may form a space in which a water tank is mounted. At least one spin mop is rotatably provided at a lower side of the main body, moving the main body by rotating, and configured to clean a floor using water in the water tank. A supply nozzle is provided at one side of the water tank housing, and when connected to a discharge nozzle of the water tank, communicates with the discharge nozzle to supply water stored in the water tank to each of the spin mops. The robot cleaner further includes a locking device rotatably provided at a position spaced apart from a circumferential surface of the water tank housing, and configured to secure the water tank in the water tank housing when mounted or to release the water tank by pushing backward on the water tank.