A reactor for hydrothermal oxidation treatment of an organic material in a reaction medium, comprising: a confinement member housed in a chamber and defining a confinement zone and a peripheral zone; at least one inlet for an oxidising fluid into the peripheral zone; a first cooling system, with an external circulation cold loop having a fluid inlet and outlet, opening into a first portion of the peripheral zone; a heating system, with an external circulation hot loop having a fluid inlet and outlet opening into a second portion of the peripheral zone; a channel with a mouth, a channel stirring system; a second cooling system for creating a temperature gradient along the channel between a cold temperature and the reaction temperature; each circulation loop being equipped with a circulator and with a heat exchanger.