The invention relates to a device for dispensing a liquid additive into a fuel circulation circuit for an internal combustion engine, comprising a reservoir (12) of liquid additive allowing an additive to be distributed into the fuel circulation circuit by means of a distribution line (16), characterized in that it comprises an additive chamber (22) in communication with the fuel circulation circuit and at least one wall (32) that is mobile and fluidtight between said additive chamber and the additive reservoir which, on the one hand, provides a fluidtight division and, on the other hand, maintains equal pressure between the additive in the additive reservoir and the fuel in the additive chamber.