Sustainable biopolymer films are increasingly explored as environmentally friendly alternatives to petroleum-based plastic materials. Regenerated silk fibroin (SF)/cellulose (CEL) composite films were prepared via an ionic liquid (IL) dissolution to enable co-processing of two otherwise difficult-to-blend biopolymers. Bombyx mori SF and CEL were dissolved separately in 1-butyl-3- methylimidazolium chloride (BMIMCl) and then blended at an elevated temperature before casting; the films were regenerated by freezing and methanol coagulation followed by extensive washing to remove residual IL. Formulations spanning 0–40 wt% CEL (SF100–SF60/CEL40) were evaluated using attenuated total reflectance Fourier transform infrared spectroscopy, ultraviolet–visible spectroscopy, water contact angle (WCA), scanning electron microscopy (SEM), and tensile testing. Spectroscopic analysis revealed that all regenerated films retained dominant silk I/random-coil features, with no significant β-sheet formation after coagulation, indicating that CEL hindered conformational transitions within the fibroin network. Optical measurements confirmed semi-transparent films; at 550 nm, transmittance decreased across the blend series from 56.06% (SF90/CEL10) to 44.59% (SF60/CEL40), while SF100 showed 34.94%. Surface wettability increased with increasing CEL content, as WCA decreased from 49.8° (SF100) to 43.3° (SF60/CEL40). SEM micrographs reveal composition-dependent texture and microdomain development without formation of large aggregates, consistent with effective dispersion after IL co-dissolution. The tensile strength declined from 3.56 ± 0.32 MPa (SF100) to 1.87 ± 0.44 MPa (SF60/CEL40), whereas stiffness peaked at SF80/CEL20 with a Young’s modulus of 113.65 ± 42.01 MPa. Overall, BMIMCl processing provides a practical route to SF/CEL films with controllable transparency, hydrophilicity, and stiffness, while also highlighting a strength– stiffness trade-off at higher CEL contents.