The physicochemical and sorption properties of the T-3K sorbent and the possibility of its use for the concentration of 90Sr were investigated. It is shown that when the concentration of calcium in the solution is more than 50 mg/l and sodium is more than 0,0001 mol/l, the strontium distribution coefficient of the T-3K sorbent is at least 104 ml/g, which allows the sorbent to be used to extract strontium from natural waters of various mineralization. A highly sensitive method for determining 90Sr in natural waters using the T-3K sorbent has been developed, the yield of 90Sr in the concentrate is 91±1%, the minimum detectable activity is 0,05 Bq/l. The developed method was tested for radioecological monitoring of natural waters in the territories of the Sverdlovsk and Chelyabinsk regions, as well as water samples taken in the sanitary protection zone of the radioactive waste storage facility of the Sverdlovsk branch of the Ural Territorial District branch of the Federal State Unitary Enterprise "Federal Environmental Operator".