A device to supply an internal combustion engine with water comprising one single tank of an exhaust system provided with exhaust gas after-treatment; the tank is divided, on the inside, into a first powder urea storage area and a second water collecting area, and wherein the powder urea and the water are mixed inside the tank so as to obtain a water solution of urea with a variable concentration, the device comprises at least one second electromagnetic injector, which is designed to inject water into the internal combustion engine, and a second pumping device, which is drowned inside the tank and draws from the second water collecting area in order to supply water under pressure to said at least one second electromagnetic injector.