The present invention relates to an apparatus and a method for treating ballast water, and an exemplary embodiment of the present invention provides a ballast water treatment apparatus including: a tank (110); a spray nozzle (121) which supplies ballast water in the form of droplets into the tank; and a gas circulation unit (130) which supplies inert gas to the spray nozzle, and sucks inert gas that is not dissolved in the ballast water in the tank.
