In this study, an ordered mesoporous silica modified with lanthanum oxide was synthesized using diatomite as silica source and applied for adsorption of phosphate from aqueous solution. By taking cost-effectiveness for practical application into consideration, the adsorbent with a theoretical La/SiO2 molar ratio of 0.2 (La0.2M41) possessed a promising performance. In the batch adsorption tests, the adsorbents with La2O3 loading possessed markedly enhanced adsorption capacities. Phosphate uptake by La0.2M41 was pH-dependent with the highest sorption capacities observed over a pH range of 3.0–6.0. Coexistent anions displayed an adverse effect on phosphate adsorption following the order of CO32− > F− > NO3− > Cl− > SO42−. In the kinetic study, phosphate adsorption onto La0.2M41 followed the pseudo-second-order equation better than the pseudo-first-order, suggesting chemisorption. The Langmuir isothermal model well described the adsorption isotherm data, showing a maximum adsorption capacity for phosphate of up to 263.16 mg/g at 298 K. In a real treated wastewater effluent with phosphate concentration of 2.5 mg P/L, La0.2M41 efficiently reduced the phosphate concentration to 28 µg P/L.