Lithium–sulfur (Li–S) batteries have received significant attention due to the high theoretical specific capacity of sulfur (1675 mA h g^–1). However, the practical applications are often handicapped by sluggish electrochemical kinetics and the “shuttle effect” of electrochemical intermediate polysulfides. Herein, we propose an in-situ copolymerization strategy for covalently confining a sulfur-containing copolymer onto reduced graphene oxide (RGO) to overcome the aforementioned challenges. The copolymerization was performed by heating elemental sulfur and isopropenylphenyl-functionalized RGO to afford a sulfur-containing copolymer, that is, RGO-g-poly(S-r-IDBI), which is featured by a high sulfur content and uniform distribution of the poly(S-r-IDBI) on RGO sheets. The covalent confinement of poly(S-r-IDBI) onto RGO sheets not only enhances the Li⁺ diffusion coefficients by nearly 1 order of magnitude, but also improves the mechanical properties of the cathodes and suppresses the shuttle effect of polysulfides. As a result, the RGO-g-poly(S-r-IDBI) cathode exhibits an enhanced sulfur utilization rate (10% higher than that of an elemental sulfur cathode at 0.1C), an improved rate capacity (688 mA h g^–1 for the RGO-g-poly(S-r-IDBI) cathode vs 400 mA h g^–1 for an elemental sulfur cathode at 1C), and a high cycling stability (a capacity decay of 0.021% per cycle, less than one-tenth of that measured for an elemental sulfur cathode).