The spatial and seasonal dynamics of ¹³⁷Cs were investigated from 2021 to 2023 in Matsukawa-ura lagoon, a semi-closed estuarine area approximately 40 km north of the Fukushima Daiichi Nuclear Power Plant, Japan. Weighted mean dissolved ¹³⁷Cs concentrations in the lagoon waters ranged from 5.3 to 19 Bq m^-3, 2.4–8.6 times higher than those in the surrounding coastal seawater and inflowing river waters. Mass balance calculation suggests that the dissolution of ¹³⁷Cs from bottom sediments sustained the high dissolved ¹³⁷Cs concentrations in the estuarine water. Furthermore, dissolved ¹³⁷Cs concentrations in the lagoon were higher in summer (July) than in winter (February). Quantification of the source revealed that dissolution caused ¹³⁷Cs concentrations in bottom sediments to decrease by more than 2.4 % over 30 days in July, but only by <0.8 % over 30 days in February. This finding indicates that warmer waters during the summer accelerate the dissolution of ¹³⁷Cs from bottom sediments.