Trim optimization is a promising strategy to improve the ship resistance characteristics and consequently increase its energy efficiency. Most of the research conducted so far regarding trim optimization is focused on unrestricted water. Since sailing in restricted water alters the flow and pressure distribution along the ship hull it is necessary to analyse the effect of trim on the total resistance in shallow and confined water. In this study, the effect of trim on the total resistance of the KCS model is analysed using computational fluid dynamics. Numerical simulations of resistance tests are performed for an even keel and four trim angles in restricted water. Numerical uncertainty for the total resistance, sinkage, and trim angle is assessed using different grid resolutions and time steps. The obtained numerical results are validated against the experimental data available in the literature. A detailed analysis of the free surface elevation, wave patterns, distribution of hydrodynamic pressure and wall shear stress, and velocity field on the symmetry plane is carried out. It is shown that a reduction of the total resistance in confined water can be achieved by adjusting the trim angle.