Starch-based nanocomposite films containing chitosan nanoparticles (S/CNP films) are biodegradable and promising alternatives for non-biodegradable synthetic plastics. Nonetheless, limited work has been conducted to investigate the biodegradability of the films in soil. Thus, this work is aimed at investigating the biodegradation of starch-based films containing different concentrations of CNP (0, 10, 15, 20, 25% w/w solid starch) via a soil burial test using compost and planting soils. The biodegradability was investigated in terms of weight loss, visual appearance, morphology, and structural changes. It was found that S/CNP films biodegraded slower than neat starch films, and the degradation rate was reduced by 46 and 44% in compost and planting soils, respectively, with the increase in the concentrations of CNP from 10 to 25% w/w incorporated into the films. The degradation rate of films in compost soil after 8 days was found to be higher (0.0617 g/day) than in planting soil (0.0266 g/day). Visual appearance, morphology, and structural change results also supported these findings. The biodegradable S/CNP films can be used to reduce the usage of synthetic plastic.