This paper proposes a novel approach to mitigate propagation losses in terahertz (THz) communication, a key enabler for future 6G networks, by integrating reconfigurable intelligent surfaces (RIS) with a proximal gradient method (PGM) optimization framework. Operating in the 0.1–10 THz range, THz communication offers ultrahigh data rates and low latency but faces challenges from severe propagation losses caused by atmospheric absorption and scattering. The proposed RIS, comprising passive reflective elements, dynamically adjusts the phase shifts to enhance signal strength and coverage. A PGM-based algorithm optimizes these shifts and effectively mitigates the losses under varying conditions. Simulations demonstrated that the PGM-optimized RIS significantly reduced path loss, lowered outage probability, and enhanced ergodic capacity, particularly over long distances. These results highlight the potential of RIS-PGM integration to address THz propagation challenges and advance high-speed low-latency communications for future 6G networks.