A product dispenser for a cistern tank, the product dispenser comprising: - a tank containing the product to be dispensed, for being fixed externally to the cistern, which has a tube for transferring said product, which reaches the bottom of the tank and an air inlet tube, which emerges upwards towards the upper area of said tank, both coupled by means of a connecting means, with homologous tubes (28,27) connected to a dosing mechanism for being placed hanging inside the cistern; - the dosing mechanism, acting each time the water inside the cistern is flushed; said dosing mechanism for being located, when installed, below the outer tank, and comprising a filling tank to be filled with the liquid to be dispensed, the filling tank being closed at the top and being linked to the tank via both the product and air inlet tubes (27,28), the filling tank being furthermore provided with a bottleneck-shaped extension below, on the outside of which a cup-shaped discharge tank is located, which is linked to a floater below, supported by a rod in the tank, this cup moving upwards and downwards, separating from the opening of the extension when the cup falls as the cistern empties, thus filling with the product to be dispensed, which it pours into the water by overflowing as it is lifted when the level of water in the cistern rises as a result the effect produced by the floater placed below it; the product dispenser comprising furthermore: - connection means between the dosing mechanism and the tank containing the product, giving continuity to the product transfer (12,27) and air inlet (11,28) tubes, located on either side of the cistern, said connection means being suitable for being fixed in an unused inlet of a conventional cistern, keeping the tank, when installed, on the outside, in a vertical position and in an arrangement so as to be filled, and the dosing mechanism held by the liquid and air circulation tubes, hanging inside the cistern and submerged in the water contained therein, when it is full. - a tank containing the product to be dispensed, for being fixed externally to the cistern, which has a tube for transferring said product, which reaches the bottom of the tank and an air inlet tube, which emerges upwards towards the upper area of said tank, both coupled by means of a connecting means, with homologous tubes (28,27) connected to a dosing mechanism for being placed hanging inside the cistern; - the dosing mechanism, acting each time the water inside the cistern is flushed; said dosing mechanism for being located, when installed, below the outer tank, and comprising a filling tank to be filled with the liquid to be dispensed, the filling tank being closed at the top and being linked to the tank via both the product and air inlet tubes (27,28), the filling tank being furthermore provided with a bottleneck-shaped extension below, on the outside of which a cup-shaped discharge tank is located, which is linked to a floater below, supported by a rod in the tank, this cup moving upwards and downwards, separating from the opening of the extension when the cup falls as the cistern empties, thus filling with the product to be dispensed, which it pours into the water by overflowing as it is lifted when the level of water in the cistern rises as a result the effect produced by the floater placed below it; the product dispenser comprising furthermore: - connection means between the dosing mechanism and the tank containing the product, giving continuity to the product transfer (12,27) and air inlet (11,28) tubes, located on either side of the cistern, said connection means being suitable for being fixed in an unused inlet of a conventional cistern, keeping the tank, when installed, on the outside, in a vertical position and in an arrangement so as to be filled, and the dosing mechanism held by the liquid and air circulation tubes, hanging inside the cistern and submerged in the water contained therein, when it is full. - connection means between the dosing mechanism and the tank containing the product, giving continuity to the product transfer (12,27) and air inlet (11,28) tubes, located on either side of the cistern, said connection means being suitable for being fixed in an unused inlet of a conventional cistern, keeping the tank, when installed, on the outside, in a vertical position and in an arrangement so as to be filled, and the dosing mechanism held by the liquid and air circulation tubes, hanging inside the cistern and submerged in the water contained therein, when it is full.