The increasing discharge of dye-containing wastewater has become a serious environmental problem, requiring efficient and sustainable treatment technologies. In this study, poultry litter–derived biochar (BC) was prepared via pyrolysis at 550 °C and subsequently modified with chitosan to produce a composite adsorbent (BC/CS) for the removal of Congo Red (CR) from aqueous solutions. The materials were characterized using SEM, BET, XRD, FTIR, and XPS analyses. BET results showed that chitosan modification slightly decreased the specific surface area from 8.28 to 8.18 m²/g and pore volume from 0.035 to 0.030 cm³ /g, while introducing abundant amine and hydroxyl functional groups on the surface. Maximum adsorption occurred at pH 3, with equilibrium achieved within 35 min. The BC/CS composite exhibited a maximum adsorption capacity of 35.36 mg/g, which is approximately two times higher than that of raw BC (17.83 mg/g). Adsorption kinetics followed the pseudo-second-order model (R² > 0.999), indicating that the adsorption process may involve chemical interactions, while equilibrium data were well described by the Langmuir isotherm model (R² > 0.99), suggesting monolayer adsorption. In competitive adsorption experiments involving CR, MB, MR, and MO dyes, the CR adsorption capacity decreased by only 7.5% for BC/CS, indicating good selectivity. Furthermore, regeneration studies showed that BC/CS retained approximately 86% of its initial adsorption capacity after six adsorption–desorption cycles. These results demonstrate that chitosan-modified poultry litter biochar is an effective, low-cost, and sustainable adsorbent for the removal of anionic dyes from wastewater.