The 4.2 ka BP Event is an abrupt climate change that might have contributed to the collapse of ancient civilizations and marks the transition between the mid‐ and late‐Holocene. Despite considerable research on this event, our understanding remains primarily based on terrestrial paleoclimate reconstructions, leaving a significant gap in understanding the role of the ocean in this event. Here, we present paired sea surface temperature (SST) and seawater δ18O reconstructions based on four fossil corals from the South China Sea. Our results demonstrate that the climate during the event was cooler, and there were meridional dry‐wet patterns in East Asia, indicating a weakened summer monsoon. Furthermore, our examination of additional coral records from the Pacific and Indian Oceans suggests that low‐latitude forcing (i.e., SST anomalies in the tropical Pacific) plays a crucial role in driving hydrology shifts in East Asia over the 4.2 ka BP interval. Plain Language Summary Around 4,200 years ago, a major climate event occurred, resulting in a global megadrought that might have caused the collapse of ancient civilizations. However, most evidence for this event comes from land‐based paleoclimate studies, leaving a gap in our understanding of how it impacted the ocean. In this study, we analyzed fossil coral samples from the South China Sea and found that the area was cooler and wetter during the event. While the cooling appears to have been consistent across East Asia, the hydrological changes exhibited a meridional pattern, with Central China also experiencing wetter conditions while North and South China were dry. By examining coral records from the Pacific and Indian Oceans, we concluded that the Central Pacific El Niño was likely occurring during this event, weakening the East Asian Summer Monsoon and causing the associated climate and hydrology changes in East Asia. Key Points Paired fossil coral Sr/Ca and δ18O records indicate a cold and wet climate in the northern South China Sea during the 4.2 ka BP event Coral and terrestrial reconstructions reveal consistent cooling but meridionally‐varied hydrological changes in East Asia around 4.2 ka BP Low‐latitude sea surface temperature anomalies in tropical Pacific significantly contribute to the shift in hydroclimate in East Asia