Abstract <p>The aim of this study is to justify regional maximum permissible concentrations (MPC_reg) of metals in transboundary rivers. Primary hydrochemical monitoring data from archival materials of the North-West Department for Hydrometeorology and Environmental Monitoring and the Neva-Ladoga Basin Water Authority were used for the calculations. The results present MPC_reg values for five metals—iron, manganese, copper, cadmium, and lead—in the Russian waters of seven rivers: Narva, Vuoksa, Seleznevka, Serga, Utroya, Lovat, and Hiitola. Three calculation methods were applied: the method of S.A. Patin, the method of D.G. Zamolodchikov, and the method of E.V. Venitsianov et al. In Patin’s method, the average concentration of each element and the standard deviation were used. Zamolodchikov’s method employed the upper and lower quartiles of the distribution. Venitsianov et al.’s method used the sample size, the upper quartile, and the standard deviation for a quantile of approximately 0.75. Based on the principle of sanitary maximalism, the method of E.V. Venitsianov et al. was substantiated as the optimal approach for calculating regional MPCs. A “very high” correlation was observed between the MPCs of metals and their clarkes in the upper continental crust.</p>