Our study provides evidence for rapid genetic and ecological differentiation of expanding populations during a contemporary range expansion. This differentiation might be caused by adaptive divergence, which could additionally fuel the success of the expansion. However, more experimental data (e.g., from reciprocal transplants or genome sequencing) will be required to rule out phenotypic plasticity as the driving force of the observed expansion. In the coming decades, an unprecedented expansion of the venomous yellow sac spider into northern Europe is expected, highlighting one of the health‐related perils of the ongoing climatic changes.