Abstract Oyster shells are a waste product from mariculture that presents a major disposal problem in coastal regions such as southeast Korea. It was found in the present study that pyrolysis of waste oyster shells under defined conditions (750 °C for 1 h under a nitrogen atmosphere) transforms this material into a sustainable reagent for efficient (up to 98%) removal of phosphates from wastewater. In comparison, raw oyster shells removed almost no phosphate from water, whereas oyster shells heated to 750 °C under an air atmosphere removed a moderate proportion (up to 68%) of phosphates from water. X-ray diffraction (XRD) analysis of pyrolyzed oyster shells showed peaks that were characteristic of calcium oxide, whereas analysis of raw oyster shells showed peaks that were characteristic of calcium carbonate. Surface morphology of pyrolyzed oyster shells also differed from that of raw oyster shells. Preliminary economic feasibility analysis indicates that cost of activated oyster shell is competitive with other wastewater treatment chemicals.