<p>We measured ¹³⁷Cs in seawater, particulate matter, and sediment cores from two coastal bays (Masan Bay and Suyeong Bay) in Korea in August 2016 and July 2017 to examine its deposition in water and remobilization in the sediments. The ¹³⁷Cs activities in the bay seawaters were similar to those in the remote ocean seawaters, suggesting that there are no extra ¹³⁷Cs inputs from land. We reconstructed the deposition history of ¹³⁷Cs using the measured distribution coefficient (<i>K</i>_d) values in the bay, the known history of ¹³⁷Cs activities in the remote ocean seawaters, and sediment ages based on the ²¹⁰Pb method. The measured inventory of ¹³⁷Cs in the sediment cores retained about 89–94% of the reconstructed ¹³⁷Cs inventory in the same depths, but the measured distribution patterns were very different from the reconstructed profiles. The measured distributions of ¹³⁷Cs were quite uniform and penetrated more deeply, with the peaks at a shallow oxic layer and around 1963, indicating significant post-depositional remobilization and redistribution due to changes in porewater chemistry with depth. The sedimentation rates (0.53–0.70&#xa0;cm&#xa0;yr⁻¹) derived from ¹³⁷Cs in the two bays agree well with those (0.51–0.68&#xa0;cm&#xa0;yr⁻¹) derived from ²¹⁰Pb. Thus, our results suggest that ¹³⁷Cs can be used as an independent geochronological tracer in coastal sediments, but careful consideration is necessary for the remobilization of ¹³⁷Cs in sediment cores.</p>