Lithium–sulfur (Li–S) batteries have garnered significant attention as promising energy storage devices due to their high theoretical specific capacity and environmental sustainability. However, several challenges including the shuttling effect of soluble long-chain lithium polysulfides (LiPSs), low electrical conductivity, and limited cycling stability hinder their practical applications. In this work, we have developed a novel organosulfur cathode material for practical Li–S batteries, featuring a selenium-doped linear sulfur chain bonded to a trithiocyanuric acid copolymer (SexS-TTCA). The incorporation of selenium at the atomic-level into long-chain sulfur significantly enhanced the electrical conductivity, lithium-ion transport, and compacted density of the organosulfur materials. Furthermore, the presence of selenium in the cathode material effectively prevents the shuttle effect by inhibiting the formation of soluble long-chain LiPSs and accelerates the kinetics of the sulfur species conversion reaction. Consequently, this novel organosulfur cathode provides a high discharge capacity of 820 mA h g−1 at 1C rate, while maintaining a low capacity decay of only 0.014% per cycle over 1050 cycles.