Strontium-90 is one of the dangerous fission products generated during electricity production in nuclear reactors and the separation of this radionuclide from contaminated water is an important step in safeguarding human health and minimizing the impact on the environment. In this research, the adsorption of strontium ions onto 3DGF-MnO₂ composites is investigated. Three-dimensional graphene foam (3DGF) was prepared by the chemical vapor deposition (CVD) method, then it was doped with MnO₂ nanoparticles by the hydrothermal method. The 3DGF-MnO₂ composite was characterized by Raman and XRD to reveal its chemical and structural properties. Raman and XRD results confirmed the functionalization of the three-dimensional graphene foam with β-MnO₂ nanoparticles. Furthermore, scanning electron microscopy showed that spindle-like agglomerated MnO₂ particles were decorated on the interconnected 3DGF surface. BJH analysis indicated that 3DGF possesses mesopores and the BET specific surface area of 3DGF was 34.01 m² g⁻¹. The effects of adsorption parameters on the adsorption process were evaluated by the response surface methodology (RSM) approach based on central composite design. The analysis of variance (ANOVA) results showed that the applied model was statistically significant due to high F (21.66) and very low P (<0.0001) values. Adsorption equilibrium isotherms were analyzed using the Langmuir, Freundlich and Dubinin–Radushkevich models. According to the Langmuir isotherm analysis, the maximum adsorption capacity of strontium ions on 3DGF-MnO₂ was estimated to be 47.39 mg g⁻¹. The mean adsorption free energy (E) was found to be 7.07 kJ mole⁻¹ using the D–R model, indicating that the adsorption process occurs by physisorption. Finally, Thermodynamic parameters such as Gibbs free energy (ΔG°), the enthalpy (ΔH°) and the entropy change of adsorption (ΔS°) were also calculated and it was found that the adsorption process was spontaneous and exothermic in nature.