Abstract: The onset of SCSSM (South China Sea summer monsoon) marks the comprehensive establishment of the East Asian summer monsoon, which is crucial for the rainy season in China. This study compares the climatological features and interannual variability of the SCSSM onset between the JRA-55, ERA5, CRA-40, and NCEP1 reanalysis datasets. Results show that atmospheric circulation and precipitation evolution are consistent among these datasets. Notably, the upper-tropospheric warm center and precipitation for the CRA-40 data are the greatest. The present study uses prevailing zonal westerly wind at 850 hPa (U850), the inversion of the mean MTG (meridian temperature gradient) in the middle and upper troposphere, and the establishment of convection in the SCS to define the SCSSM onset date. Based on different indicators, this date with the four reanalysis datasets is consistent in most years in which the circulation is completely coupled with convection. In these years, convection is established over the SCS while the upper- and lower-level circulations are adjusting. However, in individual years, the SCSSM onset date defined by the three indicators is very different because the coupling between the upper- and lower-level circulations is indefinite. When the interannual variability of the onset date defined by U850, MTG, and OLR is consistent, the SST anomaly field from April to May presents a typical ENSO-type distribution, indicating that in these years, tropical sea–air interaction is critical for the SCSSM onset. The SST anomaly in the Bay of Bengal in May is perhaps the critical factor because of which ENSO influences the SCSSM onset. When the interannual variability is inconsistent, the most apparent SST anomaly occurs along the California coast of the North Pacific, indicating that midlatitude sea–air interaction may modulate the SCSSM.