Synthetic dyes like Congo Red are commonly used in textile and batik industries, especially in Indonesia. The dyes are toxic, non-biodegradable, and pose significant environmental hazards if not treated properly prior to disposal. The removal of such dyes from aqueous solutions is feasible with conventionally activated carbon but its expensiveness and non-renewable source at the industrial level. Clam shell waste, being rich in calcium carbonate (CaCO3) content and abundantly available, presents a promising alternative precursor to produce activated carbon. The present work is focused on preparing an eco-friendly and low-cost activated carbon from waste clam shell through calcination and chemical activation with KOH. The research examines the structural modification, adsorption characteristics, and prospective applications of the resultant adsorbent in the decolorization of Congo Red dye. Clam shells were washed, dried, powdered, and calcined at 500°C. The calcined sample was activated with KOH in molar ratios from 1:1 to 1:4. The resultant activated carbon (SAC) was characterized using XRD, XRF, FTIR, and SEM. Batch adsorption was conducted, experimenting on stirring speed, contact time, and dye concentration. Adsorption kinetics and isotherms were studied through pseudo-second-order, pseudo-first-order, Freundlich, and Langmuir models. XRD and FTIR confirmed aragonite to calcite transformation and amorphous carbon structure formation during activation. The SEM images illustrated increased porosity, especially that of SAC 1:3, which exhibited a homogeneous network pore structure. SAC 1:3 realized maximum dye elimination efficiency (>99%) and adsorption capacity (1Non.9 mg/g) with minimum contact time. Kinetic data was satisfactory with pseudo-first order for SAC 1:2 and 1:3, with physisorption as the predominant mechanism, and SAC 1:4 fitted closer to the pseudo-second-order model with the implications of chemisorption. The Langmuir isotherm analysis indicated monolayer adsorption with both SAC 1:2 and SAC 1:3 exhibiting a maximum adsorption capacity of 1.87 mg/g. This research effectively proves the possibility of using clam shell waste as a high-performance, sustainable adsorbent to remove dyes. The optimized sample, SAC 1:3, presents a solution with much potential for water treatment purposes, particularly in areas with environmental issues caused by textile dye pollution.