Nanocomposites of polylactide (PLA) and bacterial cellulose nanowhiskers (BCNW) with improved properties were obtained through melt compounding. Prior to melt processing, and with the aim of improving BCNW dispersion, lactic acid oligomers (OLLA) were in situ polymerized in the presence of the nanofiller (both freeze-dried and partially hydrated). This in situ polymerization reaction enhanced the compatibilization between hydrophilic cellulose and hydrophobic PLA, even leading to chemical grafting of the OLLA onto the surface of BCNW, when this was used in a partially hydrated form. The optimized dispersion attained through this pre-incorporation strategy was confirmed by comparison with materials obtained through direct melt compounding of PLA with BCNW. Differential scanning calorimetry experiments showed that although cellulose content had not effect on melting temperatures, the degree of crystallinity was significantly affected. Addition of grafted BCNW also resulted in improved mechanical properties increasing the elastic modulus and tensile strength up to 52 and 31 %, respectively, mainly ascribed to the promotion of filler–filler and filler–matrix interactions. Moreover, the developed nanocomposites showed improvements in the water and oxygen barrier properties (measured at 80 % RH), respectively, which make them attractive for food packaging applications. This could be explained by well-dispersed nanocrystals acting as blocking agents within the polymeric matrix, reducing the diffusion through the nanocomposite films and, hence, the water and oxygen permeability. Therefore, this work offers a new route for incorporating well dispersed nanocellulose within a hydrophobic PLA matrix, overcoming the dispersion problems that this entails, especially when working with melt compounding methods.