Biodegradable polymers are highly attractive as potential alternatives to petroleum-based polymers in an attempt to achieve carbon neutrality whilst maintaining the mechanical properties of the structures. Among these polymers, polylactic acid (PLA) is particularly promising due to its good mechanical properties, biocompatibility and thermoplasticity. In this work, we aim to enhance the mechanical properties of PLA using mechanically-defibrated cellulose nanofibers (CNFs), which exhibit remarkable mechanical properties and biodegradability. Moreover, we employ fused deposition modeling (FDM), one of the three-dimensional printing methods for thermoplastic polymers, for the low-cost fabrication of the products. In this study, mechanicallydefibrated CNF-reinforced PLA matrix composites are fabricated by FDM. Their tensile properties are investigated in two printing directions (0°/90° and 45°/−45°) and discussed with regards to their microstructure and fracture surface observed from scanning electron microscopy.