A microinclusion of colloform high-purity platinum in a grain of platinum-group minerals (PGM) from the alluvial gold-bearing placer deposit in the south of Western Siberia (Russia) was detected and characterized for the first time. It is different in composition, texture, and conditions of formation from high-purity platinum of other regions described in the literature. The main characteristics of investigated high-purity platinum are colloform-layered texture, admixture of Fe (0.37–0.78 wt%), and paragenesis of Cu-rich isoferroplatinum, hongshiite, and rhodarsenide. The PGM grain with high-purity platinum is multiphase and heterogeneous in texture. It is a product of intensive metasomatic transformation of Cu-rich isoferroplatinum (Pt3(Fe0.6Cu0.4)). The transformation was carried out in two stages: 1 – copper stage including three substages (Cu-rich isoferroplatinum, copper platinum and hongshiite); and 2 – arsenic (rhodarsenide). The formation of high-purity platinum was separated in time from the formation of isoferroplatinum and was carried out by precipitation from postmagmatic solutions.