A rotor assembly is provided for a piece of rotational equipment. This rotor assembly includes a rotor disk, a rotor blade and a seal element. The rotor disk is configured to rotate about a rotational axis. The rotor disk extends axially along the rotational axis to a rotor disk end face. The rotor blade includes an attachment. The attachment attaches the rotor blade to the rotor disk. The seal element is configured to seal a gap between the rotor disk and the attachment. The seal element has a longitudinal centerline that extends along an interface between the rotor disk and the attachment at the rotor disk end face.