Unmanned Surface Vessels (USVs) face significant control challenges due to uncertain environmental disturbances, such as waves and currents. This thesis proposes a trajectory tracking controller based on Active Disturbance Rejection Control (ADRC) implemented on the DUS V2500, a seagoing USV developed by Demcon Unmanned Systems. A custom simulation incorporating realistic wave, wind, and current disturbances is developed to validate the performance of the controller. Simulated experiments are supported by further validation through field tests in the harbour of Scheveningen, the Netherlands, and at sea. Simulation results demonstrate that ADRC significantly reduces cross-track error across all tested conditions compared to a baseline PID controller, but increases control effort and energy consumption. Field trials in Scheveningen, the Netherlands, confirm a reduction in cross-track error, while revealing a further increase in energy consumption during sea trials compared to the baseline controller. Modifications to the controller are proposed to improve its efficiency, and the impacts on tracking performance are discussed.