The optical and electrical properties of TiN contacts on Si-doped GaN were investigated in correlation with their structural properties. Stoichiometric TiN films were directly deposited on 2.5 µm thick n-GaN by dc reactive magnetron sputtering at room temperature, while the stoichiometry and the structural characteristics of the TiN films were determined by in situ spectroscopic ellipsometry (SE). SE was also used for characterization of the GaN surface and for chemical etching of gallium oxide. Current–voltage measurements showed an ohmic behaviour for the as-deposited and annealed TiN/GaN samples. The specific contact resistivity was found to be 4.5 × 10⁻³ Ω cm² for the as-deposited TiN film, becoming as low as 5.9 × 10⁻⁴ Ω cm² after annealing at 400 °C. Further thermal treatment over 500 °C resulted in significant TiN oxidation and poor adhesion of the TiN film on the GaN, leading to an increase in specific contact resistivity. Transmission electron microscopy revealed structural and interfacial contact changes after high thermal treatment.