In this study, an S–St–GMA copolymer was synthesized via inverse vulcanization using sulfur (S), styrene (St), and glycidyl methacrylate (GMA), and applied as a coupling agent in styrene–butadiene rubber (SBR) composites. The chemical structure of the copolymer was confirmed by FT-IR and NMR analyses. The copolymer was incorporated into silica- and carbon black-filled SBR composites to enhance filler–rubber interactions. Chemical bonding states within the rubber matrix were examined by FT-IR spectroscopy, and crosslink density was evaluated through swelling measurements. Mechanical properties and temperature-dependent viscoelastic behavior were characterized using a universal testing machine and dynamic mechanical analysis. The results demonstrate that the S-St-GMA copolymer effectively improves the static and dynamic properties of filled SBR composites, indicating its potential as a versatile coupling agent for rubber reinforcement.