In this article, we demonstrate that Interatomic Coulombic Decay (ICD) is the dominant relaxation channel of Ne 2s inner valence vacancies in free Ne clusters, with an efficiency close to 100 %. ICD designates a novel autoionization process of a vacancy in a weakly bonded atomic or molecular cluster. Its main characteristic is the release of an electron from a site different than the original vacancy, which is mediated by ultrafast energy transfer. Results are shown for cluster sizes between approx. 50-600 atoms. A trend towards apparently increased efficiency for larger clusters may result from inelastic scattering processes inside the cluster.