Significant wave height (SWH) is vital for maritime safety and oceanographic research. This study systematically evaluated the accuracy of the multi-mission along-track SWH product distributed by Copernicus Marine Service (CMEMS) with a focus on China’s coastal waters. Validation used 2022–2024 in situ data from 30 coastal operational buoys. A spatiotemporal collocation of 25 km and ±30 min was adopted to balance sample size and matching accuracy. The SWH product from CMEMS showed good performance with an overall bias of 0.10 m, root-mean-square error (RMSE) of 0.19 m, correlation coefficient (CC) of 0.98, and scatter index (SI) of 0.13. Spatially, errors tended to be lower in the south, offshore, and in deeper waters. All satellites systematically overestimated SWH under low sea states (SWH < 0.5 m) with a mean bias of 0.17 m, which was identified as a major contributor to the overall error. This study provides a practical reference for the validation of the multi-mission SWH product from CMEMS in China’s coastal waters, and also offers a useful reference for the fusion and assimilation of multi-source satellite wave data in complex coastal environments.