High‐pressure modification of LiNiPO4 with a Na2CrO4‐type structure was obtained at ambient pressure and low temperature from a mixed LiNi–phosphate–formate precursor, LiNiPO4Hx(HCOO)x·yH2O (where x ≈ 1.2 and y ≈ 2.5). The structural and thermal characterization of the precursor and the LiNiPO4 compositions were carried out by powder XRD analysis, IR spectroscopy, and DSC analysis. Thermal treatment of LiNiPO4Hx(HCOO)x·yH2O precursors between 450 and 650 °C yields a mixture of the two structural modifications of LiNiPO4: the Na2CrO4 type and the olivine type. It was established that the obtained Na2CrO4‐type LiNiPO4 is a metastable phase, which completely transforms at 700 °C into the olivine‐type phase. The enthalpy of the phase transition is ΔH = –43.40 kJ mol–1. The mechanism of formation of the two forms of LiNiPO4 from the LiNi–phosphate–formate precursor is discussed.