Device, system and method for injecting organic matter into a reaction enclosure (2) of a reactor for treatment using a supercritical water oxidation method. The device comprises a body having an inner volume defining a pressurizing capacity (24) in which a mobile assembly (29) is arranged, e.g. a bellows for dividing the inner volume into two variable volume compartments including a compressing capacity compartment (32) and a hydraulic pressurizing capacity compartment (34). The hydraulic pressurizing capacity compartment is preferably connected to the outlet (15) of an aqueous phase liquid/gas separator (14) which separates the liquid and gaseous phases of the effluent leaving the reaction enclosure (2).