The northern coastal region of the South China Sea (SCS) is the key area for wave energy research and application. Planning for wave energy resources and equipment development depends on the accurate assessment of energy distribution and variation characteristics. Based on in situ observation data for 19 months, this paper systematically assesses the wave energy resources of three typical coastal sites in the northern SCS. The results show that wave energy resources have significant temporal and spatial variabilities. The eastern part of the SCS’s northern shore has the most energy, followed by the western part and the center part. The mean energy densities during the observation period are 2.1, 0.75, and 0.33 kW/m, respectively. The energy density is relatively high in summer, followed by winter and autumn, and relatively low in spring. For example, the mean energy densities on the northeast coast of the SCS in the four seasons are 3.1, 1.8, 1.7, and 1.2 kW/m, respectively. Based on statistics for three in situ sites, the considerable energy is mostly contributed by the sea state with a wave height between 0.5 m and 1.5 m and a period between 5 s and 9 s. This study emphasizes the importance of in situ observations for wave energy measurement in nearshore locations, and the results may provide support for the planning and utilization of wave energy in the northern SCS.