An optimized graphene/RuO2/S composite is prepared by hydrothermal growth of RuO2 particles on graphene oxide sheets as the positive electrode for rechargeable lithium-sulfur batteries. The electrode with 6.1 wt% RuO2 nanocrystals and a high sulfur content of 79.0 wt% delivers an optimal electrochemical performance with high residual capacities of 508 mAh g−1 after 200 cycles and 389 mAh g−1 after 800 cycles at 1 C with a low capacity decay of 0.054%. The RuO2 nanocrystals promote the redox reaction kinetics and facilitate the transformation of sulfur chemistry, leading to large improvements in reversibility and rate capability of the composite electrode. The density functional theory calculations signify the formation of Li–O and Ru–S bonds through chemical interactions between RuO2 and Li polysulfides while the adsorption energies between graphene and polysulfide species are much higher in the presence of RuO2 than that of the neat graphene acting alone. These discoveries support the efficient entrapment of polysulfides by the composite electrode to the benefit of enhanced cyclic stability of the battery.