Ellipsometric spectra of polycrystalline thin film tantalum nitride (TaN) have been collected and analyzed to obtain the complex dielectric function (ɛ = ɛ1 + iɛ2) and complex refractive index (N = n + ik) spectra over the photon energy range of 0.059–8.500 eV. Complex optical properties are obtained for TaN in the as-deposited state and rapid thermal annealed at 750 °C for 30 s post deposition. A parametric expression including the contribution from intraband and interband transitions along with a structural model is used and fitted to experimental ellipsometric spectra using iterative least-square regression, which minimizes the unweighted error function or mean square error to extract complex optical properties. The parametric expression developed in this work is successful in describing and differentiating the optical response of measured as-deposited and annealed TaN films and can potentially be used to analyze the optical properties of similar TaN films regardless of their deposition techniques.