Abstract Dry‐wet whiplash events profoundly impact society and the environment, yet their attribution remain poorly understood. This study reveals that the unprecedented interannual wet‐to‐dry transition (return period: 979.2 years) in South China’s 1962–1963 pre‐flood season is co‐driven by internal variability and volcanic forcing. The 1962 extreme wetness is dominated by internal variability, with atmospheric circulation explaining 96% of the variability. The 1963 extreme drought is intensified by volcanic forcing by a factor of ∼71 relative to volcanically inactive periods. The 1963 Agung eruption cools the tropical Southern Hemisphere, weakens the Hadley circulation, and triggers subsidence that worsens the drought over South China. Our findings highlight the decisive role of natural variability in driving this historic event and offer key insights for future climate risk assessment.