We present the first thermoelectric measurements on pentacene field effect transistors. We report high values of the Seebeck coefficient at room temperature between 240 and 500 micro V/K depending on the dielectric surface treatment. These values are independent of the thickness of the channel and of the applied gate voltage. Two contributions to the Seebeck coefficient are clearly identified: the expected contribution that is dependent on the position of the transport level and reflects the activated character of carrier generation, and an unexpected intrinsic contribution of 265 plus minus 40 micro V/K that is independent of the temperature and the treatment of the oxide surface. This value corresponds to an unusually large lattice vibrational entropy of 3 kB per carrier. We demonstrate that this intrinsic vibrational entropy arises from lattice hardening induced by the presence of the charge-carrier. Our results provide direct evidence of the importance of electronic polarization effects on charge transport in organic molecular semiconductors.