A robot cleaner and a method of controlling the robot cleaner are disclosed. The robot cleaner may include a body, a mop, an actuator, a battery, a tank, and a controller. The controller controls a supply of liquid from the water tank to the mop, and the supply of the liquid may be controlled in consideration of a state of the battery. Water injection may be controlled according to an expected use time of the battery, such that the mop can maintain an appropriate water content, and uniform mopping and cleaning can be performed.