This study presents a five-year (2016–2020) high-resolution record of precipitation isotopes (δ²H, δ¹⁸O, d-excess, and Δ′¹⁷O) in Seoul, South Korea. All isotopes exhibit strong seasonal cycles controlled by temperature, monsoon rainfall, and moisture source humidity. d-excess reflects source-region evaporation, while Δ′¹⁷O shows reduced seasonal sensitivity and highlights atmospheric mixing and transport processes, providing new constraints on East Asian hydroclimate dynamics.