In recent years, there has been an increasing interest to replace steel reinforcement in concrete by non-corrosive material to alleviate corrosion-related problems. Glass fiber-reinforced polymer (GFRP) bars are advocated as a potential alternative, owing to their superior physical and mechanical properties. Though, the acceptance of these materials by the construction industry is critically dependent on their long-term performance. This paper investigates the durability behavior of GFRP bars embedded in moist seawater-contaminated concrete under a sustained load of 25% of its ultimate tensile stress. Samples were conditioned for 10 months at temperatures of 20, 40, and 60°C and then retrieved for uniaxial tensile testing. However, GFRP bars conditioned at 60°C experienced creep-rupture during conditioning. As such, tensile strength retentions were measured for non-creep-ruptured bars only as a means to evaluate the long-term durability of GFRP. The microstructure of creep-ruptured specimens was characterized by employing scanning electron microscopy, Fourier transform infrared spectroscopy, and differential scanning calorimetry. Research findings showed that an increase in conditioning temperature from 20 to 40°C led to a decrease in tensile strength retention from 90 to 73% due to accelerated diffusion of water and, consequently, a higher moisture uptake. At a higher conditioning temperature of 60°C, microstructure analysis highlighted development of hydroxyl groups, plasticization and chemical degradation of the matrix, and deterioration of the fiber-matrix interface. In comparison to unloaded, conditioned GFRP samples, the presence of a sustained load promoted tensile strength loss and degradation of GFRP bars. Nevertheless, this detrimental effect was more prominent at elevated temperatures.