The South China Sea (SCS) experienced its most extreme marine heatwave (MHW) during 2023–2024, breaking the historical record in intensity and duration. The MHW event set a new record with a maximum sea surface temperature anomaly (SSTA) of +1.64°C on a basin scale. Heat budget analysis indicates that approximately 64.7% of the warming during the MHW peak phase (from October to mid-November) was driven by a reduction in latent heat flux associated with anomalously weakened East Asian winter monsoon (EAWM). The extreme SSTA during peak phase was further attributed to the long-term warming trend (39.5%), the positive Indian Ocean Dipole (IOD, 27.8%), and extreme warming in the tropical North Atlantic (TNA, 21.6%), with additional limited contribution from an El Niño. These climate modes jointly intensified the western Pacific subtropical high (WPSH), causing it to break historical records in both spatial extent and intensity, which in turn suppressed the EAWM. The prolonged MHW event triggered severe compound ecological disturbances, causing basin-wide dissolved oxygen concentration to fall to 200.4 μmol kg⁻¹ and primary productivity to a record low of 0.116 mg m⁻³. These findings highlight the synergistic impact of multi‑basin climate forcing and the severe ecosystem consequences of extreme MHWs in marginal seas.