Non-native, invasive species are often characterized by being tolerant to environmental stressors, leaving them more fit relative to native species. The red alga Gracilaria vermiculophylla originates from the NW Pacific but has recently spread along the coastlines of Western Europe, where it has become abundant in many shallow, soft-bottom estuaries. Salinity is important for the local and regional distribution of algae. The distribution of G. vermiculophylla in Europe suggests that it thrives well in hyposaline environments and that it may be more fit than some native algae under such conditions. Little, however, is known about the ecophysiology of G. vermiculophylla and it is therefore difficult to predict its spread and future distribution. Laboratory experiments with G. vermiculophylla showed that steady-state salinity above 15 psu was optimal for growth and that the growth rate was reduced at salinities below 15 psu. Variable salinity reduced the growth rate and larger oscillations were more stressful than small ones. Exposure to very low salinity (0–5 psu) was stressful for the alga and algae exposed to these low levels for 2–4 days were unable to recover fully. Gracilaria vermiculophylla did not seem to perform better in hyposaline conditions than many native, estuarine species. The present distribution of G. vermiculophylla in Scandinavia can be explained well by its response to salinity, but this may not explain its present success relative to many naturally occurring algal species.