Additive manufacturing (AM), particularly Fused Deposition Modeling (FDM), is widely used for rapid prototyping and small-batch production but remains costly in developing countries due to reliance on imported feedstock. This study developed and evaluated FilAgri, a locally fabricated AM filament incorporating Philippine agricultural byproducts—specifically rice husk ash (RHA) and coconut coir—into a polylactic acid (PLA) matrix. Composite filaments with 0%, 10%, and 20% filler loadings were compounded, extruded into 1.75 mm filaments, printed into standardized specimens, and evaluated for mechanical, thermal, printability, cost, and environmental performance. Mechanical tests following ASTM standards showed that RHA enhanced stiffness and thermal stability of PLA with minor reductions in tensile strength, while coir improved impact resistance at moderate loadings. Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) revealed that glass transition and melting temperatures remained essentially unchanged, indicating good thermal compatibility. All printable formulations achieved acceptable dimensional accuracy and print success rates on a standard FDM printer. Cost analysis indicated a 16–22% reduction in material cost per kilogram compared to neat PLA, with approximately 0.20 kg of agricultural waste valorized per kilogram of filament. Among the tested formulations, RHA- 20% emerged as the optimal blend for structural applications due to its high modulus and greatest cost reduction, while Coir-10% offered improved impact resistance and good printability for tougher prototypes. These findings demonstrate the technical feasibility and cost-effectiveness of using locally available agricultural byproducts to produce sustainable composite filaments for additive manufacturing in the Philippines, aligning with Sustainable Development Goal 12 (Responsible Consumption and Production) and supporting innovation and climate action objectives.