Regional air-sea coupling fundamentally regulates East Asian summer climate, yet its mechanistic impacts remain inadequately quantified. Using the novel Regional Integrated Environment Model System (RIEMS) with explicit ocean-atmosphere coupling, we demonstrate that sea-air interactions reduce June-July-August (JJA) precipitation by 0.1±0.40 mm day⁻¹ over East Asia during 1991–2014. This suppression stems from weakened land-sea thermal contrast, reduced evaporation in the western North Pacific, and inhibited moisture transport—mechanisms partially alleviated in future scenarios but persisting over south of the China's Yellow River. Crucially, RIEMS outperforms uncoupled WRF and global models (ENS) in simulating summer temperatures, reducing WRF's land T2max warm bias by 0.7°C through improved cloud-radiation-surface flux feedback. Without air-sea coupling, WRF exaggerates warming via insufficient cloud cover (+7.0 W m⁻² sensible heat flux bias), enhanced landward moisture advection, and distorted radiation balance. Our findings establish that regional air-sea coupling is indispensable for credible simulation of both hydroclimate and thermal extremes in East Asia under current and future climates.