The electrodynamic properties of La‐doped SrTiO3 thin films with controlled elemental vacancies are investigated using optical spectroscopy and thermopower measurement. In particular, a correlation between the polaron formation and thermoelectric properties of the transition metal oxide (TMO) thin films is observed. With decreasing oxygen partial pressure during the film growth (P(O2)), a systematic lattice expansion is observed along with the increased elemental vacancy and carrier density, experimentally determined using optical spectroscopy. Moreover, an absorption in the mid‐infrared photon energy range is found, which is attributed to the polaron formation in the doped SrTiO3 system. Thermopower of the La‐doped SrTiO3 thin films can be largely modulated from –120 to –260 μV K−1, reflecting an enhanced polaronic mass of ≈3 < m polron/m < ≈4. The elemental vacancies generated in the TMO films grown at various P(O2) influences the global polaronic transport, which governs the charge transport behavior, including the thermoelectric properties.