Bacterial cellulose nanofibers (BCNF), obtained by the mechanical disintegration of BC pellicles, were used without any surface treatment for the fabrication of poly(lactic acid) (PLA) nanocomposites by a melt compounding process. The addition of BCNF in different amounts improved both the Young’s modulus and tensile strength of PLA. A 22% increase in these properties was observed in the nanocomposite with 2 wt.% nanofibers, due to the BCNF network formed at this concentration and characterized by atomic force microscopy. BCNF addition also increased the crystallinity and thermal stability of PLA, which were evaluated by thermal analysis. Due to the high purity of BCNF and the environmental friendliness of melt processing, it was concluded that PLA/BCNF nanocomposites can be designed for biomedical field and food packaging.