Effectively recovering phosphate from wastewater streams and reutilizing it as a nutrient will critically support sustainability. Here, to capture aqueous phosphate, we developed novel mineral–hydrogel composites composed of calcium alginate, calcium phosphate (CaP), and calcium silicate hydrate (CSH) (CaP + CSH/Ca-Alg). The CaP + CSH/Ca-Alg composites were synthesized by dripping a sodium alginate (Na-Alg) solution with ionic precursors into a calcium chloride bath. To change the mineral seed’s properties, we varied the calcium bath concentrations and the ionic precursor (sodium dibasic phosphate (NaH2PO4) and/or sodium silicate (Na2SiO3)) amounts and their ratios. The added CSH in the mineral–hydrogel composites resulted in the release of calcium and silicate ions in phosphate-rich solutions, increasing the saturation ratio with respect to calcium phosphate within the mineral–hydrogel composites. The CSH addition to the mineral–hydrogel composites doubled the phosphate removal rate while requiring lesser initial amounts of Ca and P materials for synthesis. By incorporating both CSH and CaP mineral seeds in composites, we achieved a final concentration of 0.25 mg-P/L from an initial 6.20 mg-P/L. Moreover, the mineral–hydrogel composites can remove phosphate even under CaP undersaturated conditions. This suggests their potential to be a widely applicable and environmentally sustainable treatment and recovery method for nutrient-rich wastewater.