The commercial K10 montmorillonite was modified with Fe³⁺ polyhydroxy cations aiming at the enhancement of adsorption affinity towards phosphate anions, which are known to cause eutrophication in lakes and reservoirs. Polyhydroxy cations with OH^-: Fe³⁺ molar ratios of 0.75:1, 1:1 and 2:1 were investigated. The incorporation of the polycations did not alter the main conformational characteristics of the montmorillonite, as verified by surface area measurements, X-ray diffractometry, FTIR, electron microscopy and zeta potential titrations. On the other hand, the materials supporting Fe³⁺ polyhydroxy cations exhibited a significant enhancement of adsorption capacity (q_max), as determined by Langmuir isotherms, from 1.7 ± 0.1 to 2.8 ± 0.1 µmol g^-1, and the adsorption energy from 0.062 to 0.26 L mg^-1. The different ratios of OH^- to Fe³⁺ did not affect the adsorption capacities. The adsorption kinetics was best described by the pseudo 2^nd order model, with the equilibrium being approached within 120 min of contact time. The adsorption was independent of pH in a pH range between 4.6 and 8.5. The increase in ionic strength increased the adsorption capacities indicating that the formation of inner-sphere complexes prevail over electrostatic interactions in the adsorption mechanism. The materials were tested for removing phosphate from three polluted water samples with phosphate concentrations between 0.0919 and 1.211 mg L^-1. The removal was quantitative and the remaining phosphate was below the limit of detection of the analytical method. The presence of 10 mg L^-1 humic or fulvic acid did not affect the performance of the materials. Thus, we believe that the modification of clay minerals with Fe³⁺ polyhydroxy cations is suitable to produce low-cost adsorbents for phosphate.