The Langrangian model Nafta3D and the velocity calculated with the NAPOM model were used to compute the residence time of water in the Gulf of Trieste. The Nafta3D model had to be upgraded by adjusting the input of velocity fields and the initial distribution of particles. All simulations required manual calibration of the vertical turbulent diffusivity coefficient (Dv). Daily averaged velocity fields for the entire year 2012 were used in simulations. The range of possible residence times was computed using the moving sequence approach. The approximate seasonal range of residence times was determined together with the corresponding range of Dv. Within the high wind periods the residence time is about 5 days; it exceeds 20 days in low wind periods. The number of residual particles along the water column was determined in order to prove that in low wind conditions the residence time in the surface layers is significantly lower compared to the bottom layers. A simple statistical analysis was performed in order to determine possible relationship between the surface wind stress and the residence time. As the first attempt of residence time modelling in the Gulf of Trieste, the thesis represents a valuable basis for further research. Furthermore, we pointed out the necessary improvements in the Nafta3D model for more accurate computations of the residence time in the future.