The intraseasonal oscillation (ISO) in the South China Sea summer monsoon (SCSSM) and its influence on regionally persistent heavy rain (RPHR) over southern China are examined by using satellite outgoing long wave radiation, NCEP/NCAR reanalysis, and gridded rainfall station data in China from 1981 to 2010. The most important feature of the ISO in SCSSM, contributing to the modulation of RPHR, is found to be the fluctuation in the western Pacific subtropical high (WPSH), along with a close link to the Madden-Julian oscillation (MJO). Southern China is divided into three regions by using rotated empirical orthogonal functions (REOFs) for intraseasonal rainfall, where the incidence rate of RPHR is closely linked to the intraseasonal variation in rainfall. It is found that SCSSM ISOs are the key systems controlling the intraseasonal variability in rainfall and can be described by the leading pair of empirical orthogonal functions (EOFs) for the 850-hPa zonal wind over the SCS and southern China. Composite analyses based on the principal components (PCs) of the EOFs indicate that the ISO process in SCSSM exhibits as the east-west oscillation of the WPSH, which is coupled with the northward-propagating MJO, creating alternating dry and wet phases over southern China with a period of 40 days. The wet phases provide stable and lasting circulation conditions that promote RPHR. However, differences in the ISO structures can be found when RPHR occurs in regions where the WPSH assumes different meridional positions. Further examination of the meridional-phase structure suggests an important role of northward-propagating ISO and regional air-sea interaction in the ISO process in SCSSM.