This paper identifies the seasonal relationships between monsoon activities over South China (SC) in winter and summer, which can help improve seasonal predictability. A predefined unified monsoon index that can well represent monsoon characteristics in both winter and summer over SC is employed. A time series of this unified monsoon index in summer and winter, and a lead-lag correlation analysis of the unified monsoon index, found that from summer to winter, monsoon activities tend to have an out-of-phase relationship (weak summer with strong winter monsoons or strong summer with weak winter), while from winter to summer, they tend to be in-phase (weak winter with weak summer or strong winter with strong summer). The composite difference between strong and weak winter monsoons shows that in the preceding summer of strong winter monsoons, monsoon activities tend to be weak, due to the influence of developing La Niña-like events; in the ensuing summer, monsoon activities tend to be strong, modulated by an anomalous cyclone over the western North Pacific (WNP), which is triggered by that La Niña-like pattern over the East Pacific. From summer to winter, the Indian Ocean dipole (IOD) mode is evident over the Indian Ocean (IO); from winter to summer, the Indian Ocean Basin (IOB) mode is dominant, which confirms the important role of the IO in these seasonal relationships of monsoon activities. A negative IOD pattern acts together with La Niña-like forcing, enhances northerly anomalies over East Asia, strengthens winter monsoon winds, and thus enhances the out-of-phase relationship. IOB cooling can capture the anomalous signal of El Niño–Southern Oscillation (ENSO) and favor the persistence of an anomalous cyclone over the WNP until the following summer (“capacitor effect”), which is important for the in-phase relationship. These roles are also further verified by numerical experiments that prescribe ENSO-like, IOD-like, and IOB-like heat source anomalies over the Pacific and IO in an anomalous atmospheric general circulation model.