Phosphorus recovery using adsorption methods has the potential to address both the problem of phosphorus accumulation in aquatic environments and the shortage of phosphorus resources. Fossil corals (FCs), which are the skeletal remains of dead corals composed of calcium carbonate, have potential as phosphorus adsorbents. In this study, we investigated the processing conditions for FCs that were optimized for phosphorus recovery. The main component of FCs is calcium carbonate (calcite), which thermally decomposes at approximately above 800°C to form calcium oxide. The specific surface area decreased with increasing temperature up to 800°C and then increased significantly at 900–1000°C. The amount of phosphate ions adsorbed was the highest for FCs calcined at 1000°C and lowest for FCs calcined at 800°C, showing a trend similar to that of the specific surface area. After phosphate adsorption, brushite was formed on all adsorbents, and hydroxyapatite additionally formed on FC900 and FC1000. The amounts of phosphate ions adsorbed by FC–FC800 showed a decreasing trend with increasing initial pH, whereas FC1000 exhibited the opposite trend. These results suggest that the main recovery mechanism of FC–FC800 is electrostatic adsorption, while that of FC1000 is calcium phosphate precipitation, and that of FC900 involves both mechanisms. These findings demonstrate that FCs are promising raw materials for phosphorus recovery and for sustainable recycling as a calcium phosphate resource.