The invention relates to a device for the sterilization of ballast water on ships by means of UV radiation, with a pump line by means of which ballast water can be taken up and discharged, wherein the pump line is passed through by a number of UV-transparent sheath pipes arranged one behind another in the direction of the pump line and in which UV radiators are arranged for the emission of UV radiation into the pump line and sheath pipes arranged one behind another in the circumferential direction of the pump line are offset by an angle α in relation to one another.