Abstarct: Abstract In the context of sustainable design and the use of recycled materials, this study investigates the effect of temperature variations on the mechanical properties of concrete beams reinforced with recycled PET fibers. The main objective is to evaluate the impact of temperature on energy absorption, flexural strength, and natural frequency of these beams. Concrete samples with two strength classes, C25 and C30, containing 0.45% PET fibers by weight, were subjected to standard tests within a temperature range of -20 to 45 °C. The results indicated that C30-PET concrete outperformed C25 in all evaluated metrics. At 45 °C, C30-PET showed an energy absorption of 2543 J (30% improvement over control), a 25% reduction in natural frequency (compared to 33.3% for C25), and a 13.6% increase in flexural strength. Additionally, the fibers improved cracking mechanisms, although their performance was moderately reduced at sub-zero temperatures (75% reduction in C25). These findings suggest that C30-PET concrete is well-suited for tropical regions. The study highlights the potential of hybrid fiber combinations and new technologies to enhance performance. The development of national standards baxsed on these results is essential for designing sustainable and climate-resistant structures. This research contributes valuable insights into the mechanical properties of concrete beams under varying environmental conditions, aiding the design and construction of durable, high-performance concrete structures.