Concrete is a primary material in construction, impacting structural integrity through its strength characteristics. With increasing global development, the demand for concrete rises, alongside concerns about its environmental footprint and native properties like brittleness. Moreover, the rapid increase of plastic waste, particularly Polyethylene Terephthalate (PET), poses a substantial environmental challenge. Because of this, waste plastic fibers derived from PET were employed in this study to modify concrete. The addition of these fibers is anticipated to enhance concrete's toughness, reduce cracking, and provide a sustainable avenue for waste recycling. Many tests were performed on the concrete mixtures once prepared. Based on the material properties and the aim to evaluate mechanical strength, 100mm x 100mm x 100mm cubes were used to measure compressive and split tensile strength; also, 100mm x 100mm x 450mm beams were used to measure flexural strength. Before mechanical testing, each of the samples, adding waste plastic fiber at varying percentages (0%, 0.12%, 0.24%, and 0.36%), was cured for 7, 14, and 28 days. Study on the investigation of the effect of these waste plastic fibers in concrete mixtures for the specified curing times and percentages discovered that their addition increased compressive strength (up to 29.99 MPa, a 19.3% increase with 0.36% fiber), improved flexural strength (up to 6.65 MPa, a 10.46% increase with 0.24% fiber), and also showed improvements in split tensile strength. In summary, waste plastic fibers can effectively enhance concrete's mechanical performance, offering a sustainable construction solution by reusing PET waste and improving material properties.