The fabrication of sustainable packaging films based on chitosan/starch (CTS/Starch) blends, reinforced with Chitosan Nanoparticles (CNPs), was achieved via the casting blend technique. This research explored the impact of varying CNPs loading on critical physicochemical properties, including water vapor permeation (WVP), thermal stability, and mechanical strength. To elucidate the structural and chemical complexities of the blend films, surface morphology was investigated via Scanning Electron Microscopy (SEM), internal architecture was visualized using Transmission Electron Microscopy (TEM), and molecular interactions were probed through Fourier Transform Infrared (FTIR) spectroscopy. The reduction in WVP from 6.18 ± 0.54 to 5.38 ± 0.93 g.m⁻¹.s⁻¹.pa⁻¹, equilibrium moisture content (EMC) from 16.52 ± 1.03% to 12.5 ± 1.05%, and water absorbency (WA) from 340 ± 1.63% to 88.65 ± 1.12% in CTS/Starch blend films demonstrated loaded with (0–8 wt%) CNPs loading. Concurrently, films with 2–8 wt% CNP loading exhibited an increase in opacity from 2.38 ± 1.01 mm⁻¹ to 4.83 ± 0.83 mm⁻¹, accompanied by a decrease in transmittance from 89.20 ± 0.50% to 79.70 ± 1.20%. These findings collectively indicated that the CNP-incorporated chitosan/starch composites offer enhanced ultraviolet light shielding and improved water barrier capabilities compared to the non-reinforced chitosan/starch films, underscoring their promising utility in food and pharmaceutical packaging applications.