We report a simple, one-pot hydrothermal method for synthesizing tin oxide (SnO₂) nanoparticle-decorated reduced graphene oxide (RGO) nanocomposites. Transmission electron microscopy images show that flower-like SnO₂ architectures are homogeneously dispersed onto the RGO surface. The composite exhibits a maximum specific capacitance of 396 F g⁻¹ (at a current density of 4.5 A g⁻¹), with a capacitance retention of 92.6% even after 10 000 continuous charge/discharge cycles, which is sevenfold higher than that of pure RGO (55 F g⁻¹). The high capacitance is attributed to the synergetic effect of SnO₂ and RGO. In addition, the composite shows efficient photodegradation of methylene blue under visible light. The enhanced degradation is ascribed to good adsorption and improved separation efficiency of photo-induced electron–hole pairs.