A rotor for a thermal turbomachine, in particular a gas turbine, is configured for guiding a medium in its interior. In order to guide the medium in the interior with low flow losses, a pair of rotor disks lie on one another, of which the first of the two rotor disks of the pair has holes, in order to feed the medium from the outside into the rotor interior, and rib-shaped ribs, in order to guide the medium further in the direction of the rotational axis, and the second rotor disk of the pair being configured with the aid of an auxiliary web in such a way that the flow passages which are formed between the ribs as viewed in the circumferential direction are delimited axially at least over the large part of the radial extent of the ribs.