<p>Determining the mechanical properties of thin films presents significant challenges due to their nanometer-scale thickness. The separation of thin films from their substrates for standard plastometric testing is often difficult, if not impossible, complicating the direct measurement of their properties. Consequently, nanoindentation tests, which involve using small indenters and analyzing force-displacement curves, are commonly employed to assess the mechanical properties of thin films. However, experimental methods alone may be insufficient for accurately determining these properties for such thin films. This paper proposes an approach that combines numerical modelling of nanoindentation tests with the finite element method and inverse analysis to determine the optimal material constants for the substrate and thin film. The study focuses on TiN thin films deposited on silicon and stainless steel substrates as case studies. Prior to extracting the properties of the thin films, a comprehensive numerical accuracy analysis of the nanoindentation model was conducted. This involved investigating the impact of the digital model on the accuracy of results, comparing 2D and 3D models to optimize computational efficiency, and analysing the effect of finite element mesh discretization. The critical importance of accurately representing the indenter shape for reliable results was also highlighted. Following model validation, a series of nanoindentation simulations were performed on silicon and subsequently on the TiN/Si structure, enabling the separate determination of material constants for the substrate and the TiN thin film. The procedure was then applied to the TiN/SS structure for verification. The findings demonstrate that this approach enables the determination of the as-deposited thin film material properties based solely on nanoindentation tests and a robust numerical model, and it can be extended to other thin films.</p>