Water treatment plant (WTP) sludge was used as a low-cost adsorbent for manganese (Mn(II)) removal from aqueous solutions. The sludge was subjected to thermal treatment (80–500 °C) and chemical activation with KOH to enhance its physicochemical properties. The activated material exhibited increased porosity and surface functionality, reaching a maximum specific surface area of 191.29 m²/g. Batch adsorption experiments were performed to evaluate the effects of pH, contact time, temperature, adsorbent dosage, and initial Mn concentration. Mn removal was strongly influenced by pH and contact time, with improved performance under alkaline conditions. Adsorption equilibrium was best described by Langmuir model, indicating monolayer adsorption, while kinetic data followed a pseudo-second-order model, suggesting chemisorption as the rate-limiting step. The maximum adsorption capacity reached 0.35 mg/g. Leaching tests demonstrated low release of aluminum, iron, and manganese, confirming the environmental safety of the material. Although the adsorption capacity is moderate, the results highlight the potential of WTP sludge as a sustainable alternative for Mn removal, supporting circular economy principles.