This study investigates the hygrothermal effects of saline environments on Fiber Reinforced Polymers (FRPs), focusing on the moisture absorption, mechanical performance, and molecular degradation of Glass FRP (GFRP) and Basalt FRP (BFRP) rebars and vinyl ester resin samples (FRP matrix). Specimens were immersed in a saline solution at 60 ℃, with moisture absorption and desorption monitored over time. Mechanical testing was conducted on the FRP bars, while both mechanical testing and Fourier-transform infrared spectroscopy (FTIR) were employed to analyze the mechanical and molecular changes within the resin matrix. The results showed that resin mass saturation was reached within the first 10–15 days, and highlighted the need to reconsider how decreased oven-dried mass is dynamically considered in relation to moisture change. Pre-saturation hygrothermal exposure significantly impacted resin strength and stiffness, with most reduction taking place within the first 6 to 24 h. While stiffness loss was reversible upon proper drying, the reduction in resin strength was largely irreversible, linked to the leaching of resin components as observed in FTIR analysis. The study further highlighted the superior resistance of epoxy-based BFRPs compared to vinyl ester-based FRPs, as the former exhibited lower susceptibility to degradation. These findings emphasize the importance of considering pre-saturation environmental interactions in the durability assessment of FRPs, with implications for long-term structural performance in harsh environments.