Textile wastewater contaminated with both cationic and anionic dyes remains a major environmental challenge. This study aims to evaluate the adsorption performance of a biopolymer-based cellulose–zeolite composite membrane as an environmentally friendly adsorbent for methylene blue (MB) and remazol red (RR). The membrane was fabricated by incorporating cellulose acetate with natural zeolite and subsequently applied in batch adsorption experiments. Membrane stability under different pH conditions and surface functional groups were evaluated using FTIR analysis. The adsorption behavior was investigated by examining the effects of solution pH, contact time, and initial dye concentration, followed by analysis using Langmuir and Freundlich isotherm models. The results demonstrated excellent agreement between experimental data and both isotherm models. The maximum adsorption capacities calculated using the Langmuir model were 0.94 mg g⁻¹ for MB and 0.29 mg g⁻¹ for RR, demonstrating preferential adsorption toward dyes with different charge characteristics. The adsorption of MB was governed by electrostatic interactions, whereas RR adsorption was governed by hydrogen bonding and physical interactions. These results demonstrate that the cellulose–zeolite composite membrane exhibits selective adsorption behavior and confirm its effectiveness as a sustainable adsorptive material for the treatment of complex textile wastewater.