Abstract <p>The concentration of ¹³⁷Cs originating from the Chernobyl accident in the Caspian water was estimated by subtracting the concentration of global ¹³⁷Cs from the total concentration of ¹³⁷Cs radionuclides in water at the monitoring date (1996). Late monitoring of the Caspian waters in 1996 made it impossible to determine the “Chernobyl” ¹³⁷Cs from the ¹³⁴Cs/¹³⁷Cs ratio because of the decay of the short-lived radionuclide. The concentration of global ¹³⁷Cs in the seawater was calculated using the sorption–diffusion model of the ¹³⁷Cs absorption by bottom sediments with sorption coefficients (<i>K</i>_d) and diffusion coefficients (<i>D</i>) equal to 2700 L/kg and 1.0 × 10^–7 cm²/s, respectively. By the monitoring date (1995–1996), the inventories of global and “Chernobyl” ¹³⁷Cs in the seawater were determined to be 312.6 and 93.8 TBq, respectively, with “Chernobyl” ¹³⁷Cs accounting for 23% of the total amount. With low fallout of “Chernobyl” ¹³⁷Cs on the Caspian Sea, ¹³⁷Cs of global fallout dominated in the marine pollution. During the 56-year exposure (1964–2020), the concentration of global ¹³⁷Cs in the Caspian water decreased from 23.7 to 1.8 Bq/m³, which is typical of the current level of ¹³⁷Cs in waters of the world ocean.</p>