This thesis introduces a multi-fidelity approach to optimize ship hull design, leveraging both low-fidelity (LoFi) and high-fidelity (HiFi) viscous Reynolds-Averaged Navier-Stokes (RANS) simulations. This study primarily aims to devise a method that reduces both computational cost and time in CFD-based hull design optimization, particularly when viscous effects are significant and the Boundary Element Method (BEM), though less computationally intensive, falls short in accuracy and reliability. Traditional approaches in hull design optimization predominantly rely on either BEM or HiFi RANS simulations. While these methods are accurate, they are either limited in use cases or computationally expensive and time-consuming. This research proposes a framework where a Pareto optimization using a Multi-Objective Genetic Algorithm is conducted using a Multi-fidelity model which combines high-fidelity models and LoFi models in order to achieve accuracy at a reasonable cost. The validation of the optimization results is performed using a reference study, a Pareto optimization using the sea trial and tow tank validated marine CFD reference simulation setup by FINE™/Marine. This validation ensures the robustness and reliability of the simulation results, particularly in the absence of experimental data. Illustrated through case studies, the thesis explores the optimization of hydrofoils and multi-hull hulls. In such scenarios, a comprehensive analysis of the flow is required. Drawing upon existing literature, viscous flow base methods are identified as a more appropriate and accurate technique for modelling turbulent, free-surface flows which underscores the necessity of using CFD over BEM. Moreover, as mentioned above, the research integrates a multi-objective optimization framework (Pareto optimization), considering various performance criteria such as hydrodynamic resistance and sea-keeping qualities. This has been an important innovation for the field, and must be kept in this approach which uses Dakota, the optimization platform for this study. This thesis contributes to naval architecture by offering a pragmatic and efficient approach for hull design optimization, further enabling the development of rapid, cost-effective, and environmentally sustainable marine vessels.