PURPOSE: The brown marmorated stink bug, Halyomorpha halys (Hemiptera: Pentatomidae), is an invasive and a highly polyphagous species with a strong dispersal capacity. Unfortunately, there is currently no effective control method that can prevent or reduce the economic loss caused by this pest. Among natural enemies, microsporidia cause infections in insects so that they can generally shorten life span, reduce fertility and inhibit growth. METHODS: Current study involved field study, light and electron microscopy and molecular phylogenic analyses of a microsporidian pathogen in the populations of H. halys in Türkiye. RESULTS: The microsporidian infection were mostly observed in malpighian tubules and fatty tissue filled with high spore density. Fresh uninucleate spores are oval, measued as 3.73 × 2.01 µm in dimensions. The mature spore wall is relatively thick and measures 75-85 nm and consists of a clear endospore (40-50 nm) and an electron-dense, uniform, thin exospore (25-30 nm). The polar filament is isofilar, 85-110 mm in diameter and has 7-8 coils. It is found to be most closely related to Nosema maddoxi isolate Mn.6 isolated from H. halys in Georgia in phylogenetic tree. CONCLUSION: The spore morphology and structural features of the isolate from H. halys identify it as Nosema maddoxi. The phylogenetic analyses confirm both light and electron microscopic observations. This is the first microsporidian record from H. halys and also from the order Hemiptera in Türkiye. It is also confirmed that the invasive pest, H. halys carries its natural pathogen, N. maddoxi to new geographical locations during its distribution.