Glass fiber-reinforced polypropylene (GFRPP) bars exhibit superior corrosion resistance, secondary formability, and recyclability, making them ideal material to address steel bar corrosion issues in marine engineering. In order to rationally design the durability of GFRPP bar-reinforced structures, the long-term tensile properties of GFRPP bars under different corrosive environments (distilled water, artificial seawater and sea-sand concrete pore solution), temperatures (25?°C, 40?°C, and 60?°C), and exposure ages (28, 56, and 84 days) were systematically investigated. The macro? and micro?morphologies and chemical structure changes of the corroded bars were analyzed, the tensile performance degradation laws and discreteness were clarified, and the tensile degradation mechanisms were revealed. The results show that the tensile failure mode of corroded GFRPP bars is fiber filament fracture in all cases. As temperature and exposure age increase, the failure mode gradually changes from broom?like dispersed fracture to bundled?sheet fracture. The tensile strength and elongation at break decrease significantly with increasing corrosion temperature and exposure age. After 12 weeks of corrosion at 25 °C, 40 °C, and 60 °C, the tensile strength retention rates in simulated seawater and sea?sand concrete pore solution are 63.1%, 48.8%, and 32.4%, respectively, which are notably lower than 90.8%, 78.2%, and 56.2% in distilled water. The elastic modulus remains relatively stable before and after corrosion, with a maximum degradation rate of only 4.7%. The discreteness of tensile strength increases considerably after corrosion, and the 95% confidence strength is lower than the mean strength; using the mean value would overestimate structural reliability. Fiber?resin interfacial debonding and glass fiber etching are the intrinsic mechanisms for tensile performance degradation of GFRPP bars, and the artificial seawater and sea-sand concrete pore solution accelerates such degradation. The research findings can provide direct data support and a theoretical basis for the durability design of GFRPP bars in seawater sea-sand concrete structures.