The growth of ultrathin TiN films by plasma-assisted atomic layer deposition (PA-ALD) was studied by in situ spectroscopic ellipsometry (SE). In between the growth cycles consisting of TiCl4 precursor dosing and H2–N2 plasma exposure, ellipsometry data were acquired in the photon energy range of 0.75–5.0 eV. The dielectric function of the TiN films was modeled by a Drude-Lorentz oscillator parametrization, and the film thickness and the TiN material properties, such as conduction electron density, electron mean free path, electrical resistivity, and mass density, were determined. Ex situ analysis was used to validate the results obtained by in situ SE. From the in situ spectroscopic ellipsometry data several aspects related to thin film growth by ALD were addressed. A decrease in film resistivity with deposition temperature between 100 and 400 °C was attributed to the increase in electron mean free path due to a lower level of impurities incorporated into the films at higher temperatures. A change in resistivity and electron mean free path was observed as a function of film thickness (2–65 nm) and was related to an increase in electron-sidewall scattering for decreasing film thickness. The TiN film nucleation was studied on thermal oxide covered c-Si substrates. A difference in nucleation delay was observed on these substrates and was related to the varying surface hydroxyl density. For PA-ALD on H-terminated c-Si substrates, the formation of an interfacial SiNx film was observed, which facilitated the TiN film nucleation.