<p>This study explores the characterization and development of high-performance polylactic acid (PLA) copper-reinforced composites plasticized with acetyl tributyl citrate (ATbC) for fused deposition modeling (FDM). PLA is a widely used plastic in additive manufacturing owing to its biodegradability and favorable balance of physical and environmental properties. However, its low ductility restricts broader applications. To resolve this issue, ATbC (5–20 wt.%) was introduced into the PLA matrix as a plasticizer. Extruded filaments were characterized using optical and scanning electron microscopes to assess copper powder distribution, inter-layer adhesion, and structural integrity; differential scanning calorimetry to determine critical thermal transitions; and a universal tensile machine to measure mechanical properties. The results exhibited homogeneous copper powders without evidence of agglomeration in the analyzed cross-section. A 12.4% improvement in strength by introducing 10 wt.% copper powders and 124% enhancement in ductility by adding 10 wt.% ATbC was measured. While copper content up to 20 wt.% improved strength, they negatively affected ductility and increased the risk of copper particle agglomeration. The optimal balance between strength and ductility was achieved by combining 10–20 wt.% copper and 10–15 wt.% ATbC within the matrix. In this study, the importance of customizing the mechanical properties of metal-filled filaments and the potential of PLA/Cu/ATbC filaments in the additive manufacturing industry and 3D printing applications will be discussed.</p>