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<p>This study investigated the spatio-temporal variations in activity concentrations in the Sea of Japan (SOJ) of ¹³⁷Cs and these transport process from the North Pacific Ocean to the SOJ through the East China Sea (ECS) during 2012–2016. The ¹³⁷Cs activity concentrations in the SOJ have been increasing since 2012–2013 and reached a maximum in 2015–2016 of approximately 3.4&thinsp;Bq&thinsp;m⁻³, more than twice the pre-Fukushima accident ¹³⁷Cs activity concentration of  ∼ 1.5&thinsp;Bq&thinsp;m⁻³. The ¹³⁴Cs&thinsp;∕&thinsp;¹³⁷Cs activity ratios ranged from 0.36 to 0.51 in 2016. After taking into account radioactive decay and ocean mixing, we concluded that these ¹³⁴Cs&thinsp;∕&thinsp;¹³⁷Cs activity ratios were evidence that the Fukushima accident caused the increase in the ¹³⁷Cs activity concentrations. In the North Pacific south of Japan (NPSJ), the highest ¹³⁷Cs activities in 2012–2013 were observed in water from a depth of 300&thinsp;m, the potential water density anomaly (<i>σ</i>_<i>θ</i>) of which corresponded to subtropical mode water (STMW). In the ECS, a clear increase in the ¹³⁷Cs activity concentration started at a depth of 140&thinsp;m (<i>σ</i>_<i>θ</i> = &thinsp;25.2&thinsp;kg&thinsp;m⁻³) in April 2013, propagated to the surface layers at depths of roughly 0–50&thinsp;m, reached a maximum in 2015 and decreased in subsequent years. In the ECS, the Fukushima-derived radiocaesium activity concentration in surface water reached a maximum in 2014–2015, whereas the concentration in the SOJ reached a maximum in 2015–2016. The propagation of Fukushima-derived radiocaesium in surface seawater from the ECS into the SOJ therefore required approximately 1 year. These temporal changes in ¹³⁷Cs activity concentrations and ¹³⁴Cs&thinsp;∕&thinsp;¹³⁷Cs activity ratios indicated that part of the ¹³⁷Cs and ¹³⁴Cs derived from the Fukushima accident (FNPP1-derived ¹³⁷Cs and¹³⁴Cs) was transported within several years to the ECS and then to the SOJ via STMW from the NPSJ. The integrated amount of FNPP1-derived ¹³⁷Cs that entered the SOJ before 2016 was estimated to be 0.21±0.01&thinsp;PBq, 5.0&thinsp;% of the estimated total amount of FNPP1-derived ¹³⁷Cs in the STMW. The integrated amount of FNPP1-derived ¹³⁷Cs that returned to the North Pacific Ocean through the Tsugaru Strait was estimated to be 0.09±0.01&thinsp;Bq, 43&thinsp;% of the total amount of FNPP1-derived ¹³⁷Cs transported to the SOJ and 2.1&thinsp;% of the estimated total amount of FNPP1-derived ¹³⁷Cs in the STMW.</p>