Abstract Semi‐arid regions are expanding globally through divergent pathways: in East Asia (EA) via drying of semi‐humid lands, while in North America (NA) through wetting of arid areas. This interdecadal dry‐wet reversal across the Pacific has remained mechanistically unclear. Here we demonstrate that the Pacific Decadal Oscillation (PDO) drives this climatic seesaw. Analysis of 1948–2022 data reveals the PDO explains approximately 21% of dry‐wet variance in EA and 63% in NA. We identify a trans‐Pacific teleconnection where PDO phase transitions generate key atmospheric anomalies—a summer Eurasian anticyclone and winter‐deepened Aleutian Low—that subsequently modify East Asian monsoons and westerly flows. This atmospheric reorganization alters moisture transport patterns, ultimately producing opposing hydroclimatic trends across the continents. Our work establishes a causal relationship between PDO phase shifts and interdecadal dry‐wet reversals, providing crucial mechanistic understanding for predicting long‐term hydroclimatic risks in vulnerable semi‐arid regions under natural climate variability.

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