[en] Highlights: • We determined radium and radiocesium in seawater around the Korean Peninsula in 2014. • Radium concentrations were governed by the mixing ratio of Kuroshio Current water. • ¹³⁴Cs (0.1–0.2 mBq/L) was detected at the surface of the southern Korean Peninsula. • ¹³⁴Cs level was negligible due to the low concentration in the Kuroshio Current. - Abstract: We examined the spatial distributions of ²²⁶Ra, ²²⁸Ra, ¹³⁴Cs, and ¹³⁷Cs concentrations (activities) in seawater off the western and southern Korean Peninsula in July 2014. Radium-228 (and ²²⁶Ra) concentrations in water samples varied widely from 5 to 14 mBq/L (2–4 mBq/L), showing a negative correlation with salinity, particularly at the surface off the western Korean Peninsula. This indicates that the seawaters in this area are fundamentally comprised of ²²⁸Ra-poor and high-saline Kuroshio Current water and ²²⁸Ra-rich and low-saline water (e.g., continental shelf water), with various mixing ratios. Although Fukushima Dai-ichi Nuclear Power Plant (FDNPP)-derived ¹³⁴Cs was below the detection limit (134Cs (0.1–0.2 mBq/L) was detected in waters off the southern Korean Peninsula accompanied by higher ¹³⁷Cs concentrations (1.6–1.9 mBq/L) relative to that off the western Korean Peninsula. Combined with the lower radium concentrations, the detection of ¹³⁴Cs is explained by mixing of FDNPP-derived radiocesium-contaminated Kuroshio Current water.