Novel composite of immobilized layered K_2xMnxSn_3-xS6 (x=0.5–0.95) (KMS-1) on the surface of reduced graphene oxide (r-GO), KMS-1/r-GO (KGO), has been fabricated by employing hydrothermal technique and used for the removal of radioactive Cs⁺ and Sr²⁺ ions from aqueous solutions. The KGO was characterized thoroughly with different characteristic techniques and found that the laminar KMS-1 nanoplate were immobilized on the surface of r-GO nanosheets, forming a highly porous stacked structure. Batch studies have been done for obtaining optimal experimental parameters and investigating the effect of adsorption time, initial concentration of pollutant, pH of solution, temperature, and competing anions on the removal properties. At room temperature, the maximum adsorption capacity of KGO was 338.18 mg·g^-1 and 220.12 mg·g^-1 for both Cs⁺ and Sr²⁺, respectively. Langmuir isotherm and pseudo-second-order kinetic models were fitted well with experimental data of removed Cs+ and Sr²⁺. Adsorption showed good selectivity of composite towards Cs⁺ and Sr²⁺ in the presence of co-existing ions (Ca²⁺ and Mg²⁺). The proposed adsorption mechanism involved ion exchange through the presence of K⁺ ions in KMS-1 and formation of surface ionic interactions between r-GO and ions through COOX or SX (X denoted Cs⁺ and Sr²⁺). This study highlighted the facile applicability of novel material KGO and could be used as a new adsorbent for the removal of radioactive ions.