<p>Given the global issue of excessive plastic waste generation, this study examines the fiber-matrix interaction in tropical soils with varying weathering degrees, reinforced with recycled PET fibers. The work focuses on MAN and SAN soils, which have similar particle size distributions, both in their natural compacted state and when reinforced with 0.3% PET fibers. Various tests were conducted, including XRD, XRF, amorphous extraction, compaction, permeability, triaxial, MIP, and SEM analyses. Both soils are acidic and classified as lateritic, with a predominant sand fraction composed of quartz. In SAN soil, which exhibits a lower degree of weathering, the proportion of amorphous Al and Fe is 4 and 12 times higher, respectively, compared to MAN soil, leading to a greater electrical charge imbalance. The addition of fibers influenced the pore size distribution of the soils; however, the permeability coefficient’s magnitude order remained unchanged. An increase in deviatoric stress was observed in both reinforced soils, with absorbed deformation energy increasing by 12.7% for MAN soil and 65.2% for SAN soil. This phenomenon is attributed to the reduced electrostatic repulsion of PET fibers in acidic environments, as confirmed by SEM analysis. These findings highlight the importance of understanding the interaction effects to better comprehend the hydro-mechanical behavior of tropical soils for geotechnical applications.</p>