Description: Most macroalgae and other aquatic organisms feature life cycles without a stage of dormancy and elevated stress resistance that is comparable to plant seeds. During anthropogenic transportation, they are therefore probably more exposed to stress than plant invaders, which could result in a more rigorous selection of stress-resistant phenotypes. However, selection of stress resistance during invasion processes of aquatic organisms has rarely been studied. We therefore compared the responses of native and non-native populations of the seaweed Gracilaria vermiculophylla to heat shock in common garden-type experiments. Specimens from six native populations in East Asia and from eight non-native populations in Europe and on the Mexican Pacific coast were acclimated to two sets of identical conditions before their resistance to heat shock was examined. The experiments were carried out twice—one time in the native range in Qingdao, China, and one time in the invaded range in Kiel, Germany—to rule out effects of specific local conditions. In both testing sites the non-native populations survived heat shock significantly better than the native populations. After three hours of heat shock G. vermiculophylla exhibited increased levels of heat-shock protein 70 (HSP70) and of a specific isoform of haloperoxidase, suggesting that both enzymes could be required for heat-shock stress management. However, the elevated resistance toward heat shock of non-native populations only correlated with an increased constitutive expression of heat-shock protein 70 (HSP70). The haloperoxidase isoform was more prominent in native populations, suggesting that not only increased HSP70 expression, but also reduced allocation into haloperoxidase expression after heat shock was selected during the invasion history. This selection probably happened early because the same combination of traits was detected in all realms within the non-native range.