This study developed a decision-making support system (DMSS) for automatically determining the route priority of vessels entering/exiting the waters surrounding the port monitored by vessel traffic services (VTS). The developed DMSS was combined with marine geographic information systems (GIS), automatic identification systems (AIS), and fuzzy logic theory. The simulations of case studies for the ports of Keelung and Singapore were shown to illustrate the fitness of the developed DMSS. Based on the vessel information received/transmitted by AIS within the waters monitored by VTS, the developed DMSS can compute and output the route priority values for the captains on board and the operators in VTS stations as a point of reference. In other words, each ship in the waters surrounding the port can know the route priority for entering/exiting the port. The simulation results of case studies showed that the developed DMSS seems promising. The developed DMSS could be a useful tool in improving a port's navigation safety.