The precipitation of phosphoric acid as a hydroxyphosphate from nitric acid solutions in the presence of iron(III) oxides and hydroxyoxides under hydrothermal conditions was studied. It was shown that the dephosphorization process proceeds with the formation of iron(III) hydroxyphosphate (giniite) Fe5(PO4)4(OH)3∙2H2O. The most efficient and speedy process is implemented at temperatures of 180–200 °C and above. It was found that the concentration of phosphoric acid in the solution as a result can be reduced to 0.001 mol/L and below. Lanthanide, uranium, and thorium nitrates are stable under conditions of hydrothermal dephosphorization of solutions, in contrast to iron(III) nitrates decomposing to hematite and nitric acid. The features of the studied hydrothermal process were used to increase the efficiency of direct nitric acid leaching of monazite iron-bearing ores of the Chuktukon rare metal deposit. •Phosphoric acid is precipitated from nitric solutions with the formation of giniite.•REM, uranium and thorium remain in solution during dephosphorization.•Features of the process can be used to improve the breaking down of monazite ores.