Cellulose-based composites have aroused increasing interest as potential replacements for fossil fuel-based plastics. In this work, transparent cellulose-grafted-PLA nanocomposite films were prepared by grafting polylactide (PLA) onto cellulose. PLA was synthesized by in situ ring opening polymerization from a regenerated cellulose matrix, using zinc oxide nanoparticles (ZnONPs) in 1-ethyl-3-methylimidazolium acetate (EmimAc) as solvent. A facile route was devised to modify the starting material (cellulose paper) by partial dissolution and regeneration that used an ionic liquid (IL) as a smart nanowelding agent to assemble nanometric cellulose structures. The influence of the proportion of ZnONPs (1–5 wt%) and L-lactide (LA) monomers (10–70 wt%) used on the properties of the resulting nanocomposite films was examined by comparison with an all-cellulose composite (ACC) and pure cellulose paper. Incorporating ZnONPs and PLA was found to enhance the mechanical, barrier and optical properties of the films. The maximum tensile strength and best barrier properties were those of a film obtained from 5%ZnONPs and 70%LA. FTIR spectra confirmed a new form of interaction between PLA and the regenerated cellulose matrix. Also, XRD spectra revealed a transition from cellulose I to II and an increase in the proportion of noncrystalline cellulose through partial dissolution and regeneration. Although the surface morphology of the nanocomposite films was influenced by the presence of ZnONPs and PLA chains, their color and chemical structure were not. The transparent cellulose-grafted-PLA nanocomposite films obtained are highly promising as packaging materials