Freshening of the oceans is a predicted outcome of climate change. Marine phytoplankton organisms are in general affected by salinity changes and, given their key role in oceanic food webs and geochemical cycles, it is important to investigate the response of phytoplankton species to salinity changes. Diatom species of the genus Pseudo-nitzschia can form massive and, at times, toxic blooms, because several Pseudo-nitzschia species produce the neurotoxin domoic acid. Domoic acid can cause amnesic shellfish poisoning in humans and harm animals in the marine food web. The species Pseudo-nitzschia seriata can produce domoic acid in cold-water areas, like the Arctic. Hence, it is relevant to investigate the response of P. seriata to different salinity levels. Three strains of P. seriata were exposed to four different salinity levels (15, 20, 30 and 40). None of the strains grew at salinity 15, and maximum growth rates were found at salinity 30. All three strains contained toxins at salinities 20–40, with the highest cellular content occurring at salinity 20. The peak in toxin content was related to a significantly lower growth rate. However, the higher toxin content overrode the lower growth rate, ultimately resulting in a higher toxin potential at salinity 20. In addition to domoic acid, all strains contained isodomoic acid C in surprisingly high amounts, similar to the domoic acid content.