Glass fiber-reinforced polymer (GFRP) bars are increasingly used as internal reinforcement in concrete structures because of their noncorrosive nature, lightweight, high strength, and ultraviolet resistance. The microstructure and composition changes of GFRP bars exposed to severe environmental conditions are investigated in this paper. Test specimens were placed in moist seawater-contaminated concrete for up to 15 months at temperatures of 20, 40, and 60°C. Microstructural analysis was performed using differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and matrix digestion using nitric acid. A decrease in glass transition temperature (Tg) and matrix content of GFRP bars was recorded after conditioning. Conditioned GFRP bars exhibited matrix plasticization and chemical degradation. Findings from FTIR showed an increase in hydroxyl ions with conditioning at higher temperatures. Results of SEM indicated a separation at the fiber-matrix interface and matrix deterioration in conditioned specimens.