[en] The influence of lattice strain on the refractive index and optical band gap of NaNbO₃ thin films, deposited by the liquid-delivery spin metalorganic chemical vapor deposition method, was investigated by spectroscopic ellipsometry. Epitaxial growth of coherently strained NaNbO₃ films was confirmed by high-resolution x-ray diffraction and transmission electron microscopy. Incorporated lattice strain in the films was varied by the use of the oxide substrates NdGaO₃, SrTiO₃ and DyScO₃, which exhibit lattice mismatches to NaNbO₃ with different sign, magnitude and anisotropy. The Sellmeier dispersion was employed to analyze the ellipsometry data in energy region of 1.49–2.75 eV. The refractive index at 632.8 nm of the pseudomorphically grown NaNbO₃ films critically depends on the incorporated elastic lattice strain and results in a continuous decrease from 2.46 to 2.18 by varying the in-plane strain from compressive to tensile. Band gap energies for films grown under compressive and tensile lattice strain were determined by collecting spectroscopic ellipsometry data in a larger energy range between 0.73–6.48 eV and evaluating them by the Tauc-Lorentz dispersion. We observed that for tensily strained NaNbO₃ films deposited on DyScO₃ and SrTiO₃, the band gap energies increased to $3.60\pm <!--\; ??\; -->0.01$ and $3.64\pm <!--\; ??\; -->0.02$ eV, respectively. For the compressively strained NaNbO₃ film deposited on NdGaO₃ the band gap is shifted to still higher energies ($3.80\pm <!--\; ??\; -->0.01$ eV). (paper)