Aim:This study aimed to compare the biomechanical behavior of titanium and PEEK rods using finite element analysis (FEA) in a Grade I L5-S1 spondylolisthesis model.Material and Methods:An anatomically accurate 3D finite element model of the L3–S1 spine was created from CT data of a healthy 30-year-old male. A unilateral pars defect at L5 simulated Grade I spondylolisthesis. Instrumented models with titanium and PEEK rods were subjected to five physiological loading conditions. Von Mises stresses and displacement vectors were analyzed and stress distributions on bone, screws and rods were measured.Results:Titanium rods absorbed significantly higher loads across all loading conditions (e.g., 117 N vs. 34.5 N in compression; p < 0.01), while PEEK constructs allowed greater load sharing with the bone. Screws in titanium systems experienced higher stresses, particularly in posterior shear and axial rotation. Both materials effectively reduced stress at the pars defect.Conclusion:Titanium constructs offer high rigidity but concentrate stress on implants, potentially increasing complication risk. PEEK rods demonstrated a more physiological load distribution, reducing implant stress without compromising stability. PEEK rods may serve as a biomechanically advantageous alternative to titanium in select spondylolisthesis cases. Further clinical validation is warranted.