Synthetic fibers can improve mechanical performance but their environmental drawbacks prompt the need for greener solutions. Coconut shell biochar (CBC), a biogridable material was blended with polylactic acid (PLA) to create a bio-composite filament for fused deposition modeling (FDM) 3D printing. It create eco-friendly, high-strength components. The approach leverages natural waste material, aligning mechanical enhancement with sustainability goals. A comprehensive factorial experimental design was implemented to evaluate how various parameters influence the performance of the bio-composite. The study examined how biochar content in PLA and FDM parameters, such as layer thickness and infill pattern, affected flexural strength, modulus, and strain. The results showed that incorporating CBC enhanced the flexural strength and modulus while reducing flexural strain. The optimal performance was achieved with 6% CBC content, and ANOVA revealed that the infill pattern significantly influenced flexural properties, with perpendicular and cross patterns outperforming the parallel pattern. The best parameters for improving flexural strength and modulus while minimizing strain were 0% CBC, a parallel infill pattern (0°), and a layer thickness of 0.15 mm. The experimental and validation results showed high accuracy, with mean absolute percentage errors of 1.62% for strength, 1.28% for modulus, and 0.30% for strain. These findings provide insights into optimizing bio-composite PLA filaments for FDM 3D printing, and highlight the potential applications of PLA-CBC filament in medical devices, food packaging, and disposable cutlery.