Human activities have increasingly introduced plant species far outside their native ranges under environmental conditions that can strongly differ from those originally met. Therefore, before spreading, and potentially causing ecological and economical damage, non native species may rapidly evolve. Evidence of genetically based adaptation during the process of becoming invasive is very scant however, which is due to the lack of knowledge regarding the historical genetic makeup of the introduced populations and the lack of genomic resource. Capitalizing on the availability of old non-native herbarium specimens, we examined frequency shifts in genic SNPs of the Pyrenean Rocket (Sisymbrium austriacum subsp. chrysanthum), comparing the (i) native, (ii) currently spreading non-native, and (iii) historically introduced gene pool. Results show strong divergence in flowering time genes during the establishment phase, indicating that rapid genetic adaptation preceded the spread of this species and possibly assisted in overcoming environmental constraints.