This work investigates the feasibility of using tannin extracted from discarded betel nut shells as a sustainable bio-mordant for garment dyeing applications in Bangladesh. The research was motivated by direct industrial observations in a garment washing and dyeing plant, where large quantities of synthetic mordants and auxiliary chemicals are routinely used and discharged into water bodies. A hydro-extraction method was employed to obtain tannin from locally sourced betel nut shells using enzymatic pre-treatment, mild acid treatment, and controlled thermal extraction. The extracted tannin was chemically characterized using Fourier Transform Infrared Spectroscopy (FTIR) at Bangladesh University of Engineering and Technology (BUET), confirming the presence of polyphenolic functional groups responsible for dye–fiber bonding. The bio-mordant was applied in reactive, direct, and pigment dyeing processes, and the dyed fabrics were evaluated for colorfastness properties following ISO standards at Fakir Testing Services (FTS). Test results demonstrated acceptable performance in colorfastness to washing, water, perspiration (acid and alkali), and rubbing, indicating industrial applicability. By converting agricultural waste into a value-added textile chemical, this study presents a circular, low-impact alternative to conventional synthetic mordants. The findings support the potential integration of bio-based mordants into existing dyeing infrastructure, contributing to cleaner production practices and sustainable transformation of the garment industry.