This paper presents an experimental study on the flexural behavior of reinforced concrete (RC) beams externally strengthened with a unique and sustainable type of fiber reinforced polymer (FRP), named polyethylene terephthalate (PET). Compared to conventional FRPs, PET-FRP is characterized by its large rupture strain and low elastic modulus. The flexural behavior of RC beams strengthened with one and two layers of PET-FRP was compared to that strengthened with one carbon FRP (CFRP) layer and an unstrengthened beam specimen. Test results showed that the specimens strengthened with two PET-FRP layers and one CFRP layer displayed equal and significant strength enhancement of 47% compared to the unstrengthened beam. However, the former exhibited higher ductility than the latter by 33%. Lower strength enhancement was depicted in the specimen strengthened with one PET-FRP layer. Despite that, the ductility of the PET-strengthened beam was substantial, which even outweighed that of the unstrengthened beam by 9%. The results of this study showed that PET-FRP is a promising strengthening material that improves structural performance and provides a sustainable alternative to conventional FRP strengthening systems.