The structure of the high temperature phases of KNiPO₄ has been studied by X-ray powder diffraction and polarised light microscopy. This compound undergoes two phase transitions. At 486°C it undergoes an isomorphous first order phase transition where the Ni²⁺ ions change their coordination from 5 to 4. At 581°C the crystals transform into a non-polar phase with space group Pnma.