AbstractTorymus sinensis Kamijo (Hymenoptera, Torymidae), a classical biocontrol agent of chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera, Cynipidae), was released in Croatia, Slovenia and Hungary in 2015. Following the introduction, the research of parasitism rates and population genetic indices on 40 different sites was performed to monitor and evaluate the success of establishment. The observed parasitism rates were unexpectedly high and negatively correlated with the distance from westernmost locality, while population genetic indices showed that populations of T. sinensis did not suffer from bottleneck‐induced founder effect phenomenon. Moreover, lack of genetic differentiation demonstrates that all populations share similar genetic structure, which could be shaped only by high levels of gene flow. We conclude that T. sinensis established viable and genetically diverse populations and successfully spread naturally from Italy across Slovenia to Croatia and Hungary.