Ulva lactuca was used to produce edible films containing its methanolic extract, based on cellulose (C), cellulose nanocrystals (CNC), or their mixture (C/CNC). FT-IR analysis showed hydrogen bonding interactions among the film components. The C/CNC film exhibited reduced thickness (0.058 mm), oxygen permeability (3.11 × 10⁻³ g/100 µm m² s), water vapor permeation (0.97 × 10⁻¹⁰ g mm m⁻² s⁻¹), and higher lightness (87.12) with blocking over 99% of light at 280–600 nm with low Pa⁻¹. Moisture uptake followed Peleg’s model, showing lower sorption for the C/CNC film. Antioxidant assays revealed significant differences between films and the influence of food simulants.