Seawater offers outstanding characteristics as a heat source for the heat pump because it has less seasonal temperature variation and lower freezing temperature than river water, and also maintains 5 to 10°C temperature difference from the atmosphere. In this study, an annual heating performance of the seawater-source tandem heat pump system, assumed installed in Gangneung city near the East Sea in Korea, was simulated. For a simulation, the maximum heating capacity was assumed at 350 kW. An ambient temperature at Gangneung city was calculated from the TMY2 weather data, while the seawater temperature was calculated from the regression equation based on the measurement. A heat pump performance at full-load was calculated from the regression equation, which involves refrigerant’s evaporating and condensing temperatures, based on a commercial screw compressor performance map. A part-load performance was also considered. Simulation results show that heating systems in Gangneung area operate with less than 60% HLR (heating load ratio) for 5,700 hours (65.1%) in a year of period. The results also show that an annual heating coefficient of performance (COPH) of the seawater-source tandem heat pump system is 3.02, which is 14% greater than that of an air-source single-unit heat pump system.