Abstract <p>The ¹³⁷Cs migration from the upper reaches of the Vuoksa River, Lake Saimaa, to Lake Ladoga was studied to reveal the role of the river in the Ladoga contamination with ¹³⁷Cs from the emissions during the Chernobyl accident. The ¹³⁷Cs migration in 1988–2024 led to a decrease in its concentration in the river water from 113 to ≈4.0 Bq/m³. By 2015, the ¹³⁷Cs concentration in the Vuoksa River water approached the level of 4–5 Bq/m³, corresponding to the contamination of river waters with global ¹³⁷Cs prior to the accident. The decrease in the concentration of “Chernobyl” ¹³⁷Cs in the Vuoksa River is approximated by a two-component exponential dependence with the water half-clearance times <i>Т</i>₁ = 5 and <i>Т</i>₂ = 25 years, respectively. The runoff of “Chernobyl” ¹³⁷Cs from Lake Saimaa with the Vuoksa River water in 1986–2023 amounted to 22.5 TBq. This value is comparable in the order of magnitude with the fallout of “Chernobyl” ¹³⁷Cs on Lake Ladoga in 1986 (74.1 TBq). In 1986–1988, the runoff of “Chernobyl” ¹³⁷Cs from Lake Saimaa amounted to ≈30% of the total runoff in 1986–2024. The many-year transit of ¹³⁷Cs from Lake Saimaa caused increased accumulation of ¹³⁷Cs (≈300 Bq/kg) in the profile of silt sediments of the Vuoksa River and lakes of its catchment area.</p>