This work reports on the determination and comparison of the resistivity of ultra-thin atomic layer deposited titanium nitride films in the thickness range 0.65–20 nm using spectroscopic ellipsometry and electrical test structures. We found that for films thicker than 4 nm, the resistivity values obtained by the two techniques are in good agreement. However, below 4 nm, the comparison shows an increasing difference with decreasing film thickness. A difference with a factor of 3 was found at 1.8 nm and increased up to hundreds at 0.65 nm. We attribute this significant difference to the electron scattering effects at grain boundaries and interfaces which can not be fully taken into account by spectroscopic ellipsometry measurements.