The design of control systems for dynamic positioning (DP) of marine vessels in the presence of uncertainties is a challenging task. In order to take these uncertainties into account, different control methods are applied. In this thesis, a robust control method based on sliding mode methodology is considered for DP of a marine vessel in the presence of uncertainties. To consider the effect of uncertainties in the vessel model, a nominal value for inertia and damping matrices is considered and uncertainties are presented by unknown bounded time-varying matrices. Also, because of unavailability of vessel velocities and bias term, an observer needs to be utilized. For this, a nominal model-based observer is used and the effect of uncertainties in the performance of the observer are taken into account. The stability analyses are presented for both the estimation error dynamics and the closed-loop control system. It is proved that the observation error dynamics and the error dynamics of the closed-loop control system are both globally uniformly ultimately bounded (GUUB). Also, simulations show that, the observation error is ultimately bounded and the control system is capable of positioning the vessel at desired values with acceptable accuracies. Keywords: Dynamic Positioning, Globally Uniformly Ultimately Bounded, Marine Vessel, Observer, Robust Control Method, Sliding Mode, Uncertainties