[en] Highlights: • Radiocesium vertical profiles in the southwestern Okhotsk Sea were investigated. • ¹³⁴Cs was affected by physical oceanographic processes such as advection and mixing. • Atmospheric deposition-derived ¹³⁴Cs from the FDNPP was detected in June 2011. • The ¹³⁴Cs inventory, based on the FDNPP accident date, showed a maximum value in 2017. • FDNPP-derived radiocesium was re-delivered from the North Pacific Ocean until 2017. - Abstract: We examined the vertical ¹³⁴Cs and ¹³⁷Cs concentration profiles in the southwestern Okhotsk Sea in 2011, 2013, and 2017. In June 2011, atmospheric deposition-derived ¹³⁴Cs from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) was detected at depths of 0–200 m (0.06–0.6 mBq/L). In July 2013, ¹³⁴Cs detected at depths of 100–200 m (∼0.05 mBq/L) was ascribed to the transport of low-level ¹³⁴Cs-contaminated water and/or the convection of radioactive depositions (134Cs was detected in water samples at depths above 300 m (0.03–0.05 mBq/L), and the inventory, decay-corrected to the FDNPP accident date, exhibited its maximum value (85 Bq/m²) during this period. Combining temperature-salinity data with the concentrations of global fallout-derived ¹³⁷Cs led to a plausible explanation for this observation, which is a consequence of re-entry of FDNPP-derived radiocesium through the Kuril Strait from the northwestern North Pacific Ocean to the Okhotsk Sea and subsequent mixing with the south Okhotsk subsurface layer until 2017.