The present invention relates to a ballast water treatment system and a ballast water treatment method. A ballast water treatment system according to an embodiment of the present invention comprises: a first ballast water supply pipe for receiving a supply of ballast water from a first sea chest positioned in a non-explosion-proof area of a ship; an electrolytic bath for electrolyzing the ballast water supplied from the first ballast water supply pipe; a second ballast water supply pipe for receiving a supply of ballast water from a second sea chest, which is positioned in an explosion-proof area of the ship, and supplying the ballast water to a ballast tank of the ship; a filter provided to the second ballast water supply pipe so as to filter the ballast water passing through the second ballast water supply pipe; and a third ballast water supply pipe connected to the second ballast water supply pipe so as to supply the ballast water, which has been electrolyzed from the electrolytic bath, to the ballast water which has passed through the filter.