Herein we report the details of crystal structure and phase transition behavior of a complex phosphate,K₂Th(PO₄)₂ as observed from the variable temperature powder x-ray diffraction (XRD) and neutron diffraction (PND) studies. The ambient temperature monoclinic (P2₁/n) structure was observed to remain unchanged down to 6 K, while above 973 K, it transforms to a tetragonal (I4₁/amd) structure. This structural transition occurs with a volume discontinuity of about 13.4% and reverts back to monoclinic structure on cooling but with a large hysteresis in temperature (~100 K). From the variation of unit cell parameters and structural differences as well as differential thermal analyses studies, the transition is concluded as a first order reconstructive type. The unit cell parameters of both structures gradually increases with increasing temperature but the coefficient of thermal expansion decreases appreciably after the transition (monoclinic, a_V = 49.9 × 10^-6/K in between 6 and 973 K; tetragonal, a_V = 33.6 × 10^-6/K, in between 673 and 1173 K). The details of crystal chemistry, phase transition and thermal expansion behaviors of K₂Th(PO₄)₂ are explained, and compared with other isostructural compositions.