The invention relates to a fluidised-bed reactor with multiphase separation, and to a unit for reaction by fluidisation with multiphase separation. The reactor comprises a tank with a liquid level and a liquid-gas distributor arranged at the bottom of the tank; and includes a solid-liquid-gas separation unit arranged inside the tank, above the liquid-gas distributor. The solid-liquid-gas separation unit comprises a liquid inlet arranged below the liquid level and a liquid outflow pipe arranged between the liquid level and the liquid inlet. The solid-liquid-gas separation unit has an outlet for solids arranged at the bottom of the solid-liquid-gas separation unit and a gas outlet arranged above the liquid level. In addition, the reactor includes a pump with a suction pipe connected to the liquid outflow pipe, and a discharge pipe connected to the liquid-gas distributor. Furthermore, the reactor has a gas supply unit connected to the discharge pipe by means of a gas-transporting pipe that has a Venturi; and has a gas outflow pipe connected to the tank, arranged above the liquid level. Moreover, the reaction unit comprises a first fluidised-bed reactor and a second fluidised-bed reactor that can be interconnected for the co-current, counter-current, serial or parallel circulation of at least two phases (liquid, solid or gas) that react in said reactors.