The Seebeck effect of ZnO nanowires has been investigated with the future aim to build thermoelectric devices based on nanowire arrays for energy harvesting and potential use in low-power portable electronics and autonomous sensor systems. Bundles of ZnO nanowires have been deposited on alumina substrates by a thermal evaporation process. The ZnO nanowires have been characterized by means of a purposely-developed experimental set-up, showing a negative Seebeck coefficient as for n-type semiconductors.