Glass Fiber Reinforced Polymer (GFRP) is commonly applied as a substitute for steel reinforcement because of its superior tensile strength, low density, and remarkable resistance to corrosion. However, exposure to elevated temperatures can significantly affect its mechanical performance. This study investigates the influence of thermal exposure on the tensile strength and elastic modulus of 8 mm diameter GFRP bars by conducting tensile tests on specimens heated to 100 °C, 250 °C, and 450 °C, according to ASTM D7205M-21 standards. steel bars were also examined under identical conditions. The results show that the tensile strength of GFRP decreases progressively with temperature, with the most significant reduction (27.04%) occurring at 450 °C. Similarly, the elastic modulus of GFRP declined by 8.67%. In contrast, steel bars exhibited increased tensile strength up to 250 °C and remained relatively stable in elastic modulus. Regression analysis confirmed a strong negative correlation between temperature and GFRP tensile strength (R2 = 0.963). These findings highlight the limitations of GFRP in high-temperature applications and underscore the importance of considering thermal exposure in the design of composite materials to ensure structural integrity.