In this work, the effect of various polyol‐based plasticizers, including glycerol (GLY), diethylene glycol (DEG), and polyethylene glycol (PEG), on the properties of alginate (ALG)‐based films and cellulose nanocrystals (CNC)‐filled ALG‐based bionanocomposite films was investigated. The CNC used as nanoreinforcing fillers were extracted, at nanometric scale, from sugarcane bagasse fibers using sulfuric acid hydrolysis process, and their morphology and structure were characterized. Plasticized ALG films and plasticized ALG‐CNC bionanocomposite films were prepared by solvent casting method. Before the casting process, the steady shear viscosity of film‐forming solutions of all studied formulations was studied and compared. The obtained ALG‐based films were successfully characterized in terms of their structure, transparency, water vapor permeability (WVP), moisture uptake (MU), thermal, and mechanical properties. It was found that all studied plasticized films keep the same transparency level of neat ALG films, thus suggesting that the selected plasticizers are compatible with the ALG biopolymer, and the CNC are well dispersed within the bionanocomposite films. The elastic modulus and tensile strength of neat ALG films were largely decreased after the addition of different plasticizers, while the elongation at break was significantly increased. However, these selected mechanical parameters were simultaneously improved when CNC are incorporated into plasticized ALG bionanocomposite films. The WVP and MU properties of ALG films were decreased when different plasticizers and CNC were added. Herein, the steps to form new eco‐friendly ALG films were described by taking advantage of the combination of plasticizers and CNC with ALG biopolymer. The as‐produced films exhibited good optical transparency while reducing the WVP and enhancing tensile properties, which are the main properties required for packaging applications.