A robot cleaner enables a wet cleaning, has a replaceable cleaning unit, and ensures a sufficient friction force for doing a wet cleaning of a floor. The robot cleaner includes a frame including a wheel for driving, a replaceable cleaning unit formed at a lower part of the frame to clean a bottom surface and disposed in front of the wheel, a tank disposed at an upper part of the wheel and configured to supply fluid to the cleaning unit, a pump configured to pump the fluid contained in the tank such that the fluid contained in the tank is supplied to the cleaning unit, a hose configured to extend to the pump and the upper part of the cleaning unit and through which the fluid pumped by the pump flows, and a bumper formed on a side of the frame.