The construction industry has seen a growing emphasis on the use of sustainable materials in recent years. This is driven by various factors, including a desire to reduce environmental impact, improve indoor air quality, and promote the health and well-being of building occupants. One sustainable material that is being increasingly utilized in construction is natural fibers. Phragmites australis fibers, in particular, are renewable, biodegradable, and have a low carbon footprint. The present study aims to evaluate the impact of Phragmites australis fibers on the behavior of reinforced concrete beams. Five concrete mixes were utilized in the experiment, with the control mix having a 1:1.5:3 ratio of cement to sand to coarse aggregate by weight. The other four mixes incorporated Phragmites australis fibers at 0%, 0.5%, 1%, and 1.5% of the volume of the mix, with cement replaced by 10% glass by weight. The water-to-cement ratio was set at 0.4 for all mixes. Concrete cubes, cylinders, and prisms were prepared to determine mechanical and physical properties, while reinforced concrete beams were used to assess structural performance. The results of the experiment showed that the addition of Phragmites australis fibers slightly decreased the compressive and tensile strength of the concrete compared to the control mix. However, the inclusion of 0.5% Phragmites australis fibers enhanced the split tensile and flexural strength of the concrete. In terms of reinforced concrete beams, the maximum load-bearing capacity was realized for the mix with 10% glass and 0% Phragmites australis fibers. However, the highest ductility index and deflection were achieved for the mix with 10% glass and 0.5% Phragmites australis fibers. Therefore, the use of Phragmites australis fibers can improve the structural performance of concrete.