The invention relates to a device for cooling slots in a turbomachine rotor disk, comprising a rotor disk having a plurality of slots and a flange. The device also comprises blades, each having its root mounted in a respective slot, a retaining annulus having one end mounted against the upstream radial face of the disk, and a flange disposed around the flange of the disk and co-operating therewith to define a space forming a cooling air diffusion cavity that opens out into the bottom of each of the slots, together with air admission orifices opening out into the diffusion cavity at the upstream end thereof, the end of the retaining annulus that is mounted against the upstream radial face of the disk including a plurality of openings that open out radially into the bottom of each of the slots in the disk, at the upstream ends thereof.