Evaluating the permanent deformation of soils used in pavements or final earthwork layers is essential for designing highways and railways when adopting a mechanistic approach to structural design. However, due to environmental concerns, exploiting new soil deposits for such projects has become increasingly challenging, making soil stabilization or reinforcement a viable alternative. In this context, this study sought to explore the effect of adding piassava fibers to a clayey soil commonly found in subgrade layers in Brazil. Repeated load triaxial tests were conducted to assess permanent deformation under two pairs of deviator and confining stresses: (210, 70) and (450, 100) kPa, with 100,000 loading cycles applied at a frequency of 5 Hz. Resilient modulus tests were performed following national standards, using samples of natural soil, natural soil with 1.5% piassava fiber, and natural soil with 1.5% piassava fiber and 2% cement. Results showed that natural soil exhibited high permanent deformation under the higher stress pair, while the simple addition of fibers significantly reduced deformation. With the addition of cement, total permanent deformation was minimal, indicating that piassava fiber is a promising material for reinforcing pavements or earthworks.