The plantar fascia is a tissue that attaches the heel bone to the base of the proximal phalanges; it plays a crucial role in the phases of walking and supports the foot arch. Several diseases, such as diabetes and Ledderhose, can significantly impact the biomechanical properties of the soft tissues in the foot, affecting this ligament. Different authors have reported a non-linear behavior of the plantar fascia with this type of pathology. This work aims to propose a material model for healthy, diabetic, and Ledderhose plantar fascia tissue and to compare the structural response of the plantar fascia through a numerical finite element model. Experimental data previously reported were used to obtain the stress versus strain curves for each case and to fit and select the material model. CAD models for sample tests and plantar fascia were generated. The material models selected were compared to experimental data, obtaining a mean error of 1.12 ± 4.95%, 8.36 ± 1.29%, and 16.28 ± 9.45% for healthy, diabetic, and Ledderhose plantar fascia, respectively. Applying these material models, the structural response of the plantar fascia showed differences up to 71.70% compared to the healthy case for normal stresses and up to 7.11% in strains. Our findings suggest that in cases of diabetes or Ledderhose diseases, the stress distribution increases, while strain distribution decreases in both cases regarding the healthy case.