We assess the sea level trends over 2002-2022 from three different altimeter data products using in-situ observations in the coastal zone of the northern South China Sea (SCS). Three data products are the CMEMS (Copernicus Marine Environment Monitoring Service), ESA CCI (European Space Agency Climate Change Initiative) and our SCMR (Seamless Combination of Multiple Retrackers). The results show that the trends from SCMR are more consistent with those from tide gauge records than other two altimetry products in terms of smaller root mean square (RMS, 1.89 mm yr^-1) and standard deviation (SD, 2.00 mm yr^-1). In addition, there exist systematic biases in trend estimates from different altimeter datasets with respect to the SCMR, which are 0.77 mm yr^-1 for CMEMS and −1.5 mm yr^-1 for ESA CCI. Moreover, the degradation of sea level trends occurs when the altimeter ground track passes through the large islands and headlands. Therefore, the use of altimetry products should be very careful in these places. We also find that the ENSO-related signals mainly impact on sea levels in the west of Luzon Strait and of the Philippine coast, and are dampened along the Chinese coast. The regional mean sea level trend is reduced from 3.54±0.92 mm yr^-1 to 3.18±0.84 mm yr^-1 after removing the ENSO-related low-frequency variability. At last, the variation of sea level trends from offshore oceans to coasts is nonlinear, indicating the complexity of sea level changes in the study area. The blending of tide gauge records and SCMR reprocessed altimeter data has been conducted in four tide gauges for a longer period of January 1993-April 2022, which would further contribute to the analysis of decadal variability and long-term sea level trends.

sea level rise, coastal zone, waveform retracking, radar satellite altimetry, South China Sea, tide gauges