The invention relates to a device for cooling the slots of a turbomachine rotor disk, the device comprising a rotor disk having a plurality of slots and a flange that extends upstream from a radially upstream face of the disk. The device also comprises a retaining annulus having a flange that is placed around the flange of the disk, co-operating therewith to provide a space forming a cooling air diffusion cavity, said cavity opening out into the bottom of each of the slots of the disk, and a plurality of air admission orifices opening out into the diffusion cavity at the upstream end thereof, the cavity having means at its downstream end for reducing the flow rate of air penetrating into the slots of the disk that are situated radially in register with air admission orifices.