Device for mixing and distributing a dense, generally liquid, fluid and a light, generally gaseous, fluid, placed in a reaction chamber upstream from a granular bed (70) or between two successive granular beds, the said device being characterized by a tubular system (50) for the introduction of dense fluid from outside the reactor, up to a level lower than the level of establishment of the interface between the dense fluid and the light fluid, situated above the plate and preferably more or less next to that of the plate (62). Application of this device to any type of gas/liquid reaction in a fixed bed in particular for hydrotreatment.