ABSTRACT The complementary relationship of evapotranspiration (CRE) hypothesis has been widely used to estimate actual evapotranspiration (AET) indirectly under various climatic conditions. However, its applicability in monsoon‐dominated regions remains underexplored. This study investigates the dynamics of potential evapotranspiration (PET) and AET in the Yongdam dam basin, a representative monsoon climate region in Korea, using flux tower observations and meteorological data. PET and wet‐environment evapotranspiration (WET) were calculated using the FAO Penman–Monteith and Priestley–Taylor equations, respectively. Our findings reveal significant deviations from the expected CRE pattern in monsoon regions due to the dominant influence of external air mass inflow, which disrupts the correlation between soil moisture and atmospheric humidity. These results empirically demonstrate the deviation of the CRE hypothesis in monsoon climates, emphasizing the need for region‐specific evaluations of evapotranspiration dynamics. This study provides critical insights into evapotranspiration processes, offering valuable directions for refining hydrological models and improving water resource management in dynamic climatic regions.