The brown marmorated stink bug, Halyomorpha halys, native to East Asia, is an invasive pest of economic importance. It has invaded North America and many European countries and is further expanding its range. In Belgium, it was first observed in 2011. Halyomorpha halys is known to cause severe damage in commercial fruit orchards and vegetable crops. A dramatic and unmitigated expansion of H. halys in its adventive range could lead to significant economic implications for agricultural production. In this study, occurrence data of H. halys since its first observation in Belgium was analysed together with molecular information to map the populations and evaluate the genetic diversity of this pest. The genetic diversity of H. halys in Belgium was compared to data from other invaded and native countries reported in previous studies to identify possible invasion routes. The analysis of 1176-bp of mitochondrial DNA cytochrome c oxidase I and II genes (COI and COII) led to the discovery of two novel COI-COII haplotype combina-tions currently unique to Belgium. The invasion of H. halys in Belgium is likely the result of multiple and ongoing introductions from its native region and from already invaded countries in Europe, particularly Italy. The expansion of the brown marmorated stink bug populations in Belgium is recent and ongoing. Presently, it appears to thrive best in northern Belgium.