By controlling the crystallinity and average grain size of thermoelectric NaxCoO2 thin films, a doubling of the thermoelectric power factor is achieved in combination with a strong suppression of the thermal conductivity. These structurally engineered NaxCoO2 thin films outperform single crystalline and polycrystalline samples at room temperature and demonstrate the potential of thermoelectric oxide thin films. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.