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Turbidity and color are critical parameters in industrial wastewater treatment, particularly in textile effluents containing suspended particles and dyes. Conventional coagulation methods are effective but have significant drawbacks. In this context, the development of natural coagulants has emerged as a promising alternative method. This study investigated the production of a natural-based coagulant from chestnut shells (Castanea sativa), an abundant agro-industrial residue in Portugal, through tannin extraction followed by chemical modification via the Mannich reaction. The tannin extraction process was statistically optimized using a Box-Behnken Design to evaluate the effects of pH, extraction time, and biomass dosage. The coagulant performance was evaluated using Jar Test assays for turbidity (kaolin) and color (methylene blue) removal. Statistical analysis showed that these factors affected turbidity and color removal differently, with higher extraction times and biomass dosage generally favoring turbidity removal. The results showed high turbidity removal efficiencies, reaching up to 96%, whereas color removal efficiencies were lower, reaching approximately 26%, suggesting that dye removal depends not only on coagulation mechanisms but also on specific interactions between the dye and the coagulant. Overall, the results demonstrate the potential of chestnut shell residues as a sustainable source of natural-based coagulants for wastewater treatment.