The recycling and repurposing of discarded materials plays a crucial role in advancing sustainable development. The introduction of fiber-reinforced soil for the stabilization of soft soil has become important research in the field of geotechnical engineering. Numerous studies on waste materials, such as fly ash combined in various ratios with soil, are also available. There are, however, very few applications where soil-fly ash combined with artificial fiber is used. Aiming at the efficient application of fiber-reinforced fly ash-treated soils, the optimization of the material composition was focused on in this study. Hence, this study was carried out to investigate the strength characteristics of red soil-fly ash-polypropylene fiber (RS-FA-PPF) mixes at suitable proportions. All the materials were characterized by their physical properties. The maximum unconfined compressive strength obtained for 15, 20, 25, and 30% fly ash (FA) content in red soil-fly ash (RS-FA) mixes was at 20%, later, with the addition of 0.5% to a maximum 2% polypropylene fiber (PPF) with RS-FA mixes for enhancing maximum strength. The UCS test demonstrated that the addition of PPF to RS-FA mixtures enhanced the strength compared to using soil alone, minimized post-peak strength loss, and shifted the failure behavior from brittle to ductile. The technical parameters of the red soil, fly ash, and polypropylene fiber-stabilized soil met specification standards and positively influenced the compaction characteristics and unconfined compressive strength, contributing to the sustainable development of construction practices.