<p>The increasing consumption of plastic in the world has led to severe environmental problems, particularly in the disposal of polyethylene terephthalate (PET) waste. In this regard, this paper discusses the feasibility of incorporating waste PET fibers in high-strength reinforced concrete (HSC RC) beams to enhance shear performance and promote the utilization of sustainable construction materials. Because HSC is inherently brittle, it can suddenly fail due to shear. Therefore, strategies to increase its ductility and energy absorption capacity are required. Despite reports of PET fibers improving tensile and flexural properties, the effect of PET fibers on shear strength is not clearly known. While some studies have explored PET fibers in normal-strength concrete, there is no data on the optimized interaction between specific fiber lengths and high-volume fractions in High-Strength Concrete (HSC) beams failing in shear. This study complements existing literature by evaluating the effect of PET fiber length (30 mm, 40 mm) and volume fractions (up to 1.5%) to refine predictive shear models for High-strength RC beams containing Fibers. The experimental program consisted of casting and testing 20 RC beams (120 × 150 × 1200 mm) that were to fail in shear. PET fibers of 30 mm and 40 mm were added at volume fractions of 0%, 0.5%, 0.75%, 1%, 1.25% and 1.5%. Beam tests were performed on three-point bending to identify shear capacity, energy absorption, and failure modes. The results indicated that the 30 mm PET fibers led to a decrease in shear strength up to 19.38%, and the 40 mm fibers increased shear strength by 8.16%, which reflected the influence of fiber length on shear performance. In addition, PET fibers also played a significant role in ductility and energy absorption to reduce the rate of crack propagation and improve post-peak behavior. Comparing the existing shear strength models of ACI and Eurocode, discrepancies existed, and this means that the models cannot be applied to estimate the shear strength of PET fiber-reinforced beams. Regression analysis was conducted to come up with a better shear strength prediction model, which provided a better accuracy for the PET-reinforced HSC beam. The results indicate that PET fibers can be successfully introduced into concrete to make it more sustainable, although the characteristics of the fibers should be adjusted to achieve a balance between strength and ductility. A regression equation was constructed to predict the shear strength of PET-reinforced HSC beams. The proposed regression model was found to be very accurate with R² = 0.98672, which validates its usefulness in predicting the shear strength of PET-reinforced HSC beams.</p>