Chicken eggshells are an abundant waste from the food industry, composed mainly of calcium carbonate (~96%) along with proteins, and have demonstrated potential as reinforcing agents in biodegradable polymeric matrices. Their rigid mineral structure, biodegradability, and high surface area after milling make them suitable candidates for use in sustainable packaging development. In this study, eggshell powder (ESP) was valorized as inorganic fillers for the development of poly(lactic acid) (PLA) and reprocessed PLA (rPLA) biocomposites plasticized with acetyl tributyl citrate (ATBC). ESP was obtained from post-consumer eggshells by a process involving washing, sterilization, shredding, grinding and sieving to achieve particle sizes below 45 µm. ESP microparticles were incorporated into PLA-rPLA-ATBC matrices at 1, 3 and 5 wt.%, and further processed by melt extrusion (170-190 °C) into filaments for subsequent thermoforming by hot-pressing. The mechanical, morphological and thermal properties of the resulting biocomposites were analyzed. Homogeneous and transparent ESP-loaded films were successfully obtained. The results indicate that increasing the ESP content improves the stiffness of the material, while the addition of ATBC improved processability and the flexibility of the final biocomposites. This work contributes to the circular economy by valorizing agro-industrial waste as functional additives in biodegradable polymers. This first step shows that eggshell particles as mechanical reinforcements and potential carriers of active compounds within matrices, such as active food packaging, are an attractive alternative for the development of next generation sustainable packaging materials.