The design of an offshore wind turbine (OWT) founded on a monopile foundation is principally based on a dimensioning criteria related to its fundamental frequencies. These frequencies must remain outside the excitation frequencies to avoid resonance. For the calculation of the OWT natural frequencies, several studies exist but few of them simultaneously consider both the real geometrical configuration of the OWT superstructure (tower, blades, transition piece and nacelle) and the three-dimensional (3D) soil domain and its interaction with the monopile foundation. In order to ensure accurate determination of the system frequency, a full 3D model of a 10 MW DTU offshore wind turbine installed in sand is developed and simulated using the commercially available finite element code ABAQUS/Standard. The main objective is to perform a rigorous modal analysis of the wind turbine considering the entire soil-foundation structure system. The obtained natural frequencies are compared with those corresponding to other simplified foundation models. In addition, the effect of (i) the monopile diameter and embedded depth and (ii) the sand relative density, on the system natural frequency is presented. Results indicate that when considering the soil-structure interaction the OWT’s first natural frequency is substantially decreased. Nonetheless, among the numerous foundation models found in literature, the distributed spring model based on the modulus of subgrade reaction proved to give the best estimate in terms of the first natural frequency. Finally, results have shown that there exists a critical monopile embedded depth above it there is no further increase in the first natural frequency.