Alexandrium ostenfeldii is among the most intensely studied marine planktonic dinophytes and in the last few years blooms have become a recurrent phenomenon mainly in brackish coastal waters. Since 2012, A. ostenfeldii recurs annually in the Ouwerkerkse Kreek, a Dutch brackish water creek discharging into an estuary with large stocks of mussels, oysters and cockles. The creek is characterized by highly dynamic abiotic conditions, notably salinity. Here, we investigated the impacts of salinities ranging from 3 to 34 on growth and toxin content of an A. ostenfeldii isolate from the creek. Our results demonstrate a broad salinity tolerance of the Dutch A. ostenfeldii population, with growth rates from 0.13 to 0.2 d−1 over a salinity range from 6 to 34. Highest paralytic shellfish toxin and cyclic imine toxin cell quotas were observed for the lowest and highest salinities, and were associated with increases in cell size. Lytic activity was highest at the lowest salinity, and was 5-fold higher in the cell-free supernatants compared to cell extracts. Together our results demonstrate a tight coupling between salinity and A. ostenfeldii growth rate, cell size and toxin synthesis, which may have consequences for the seasonal dynamics of bloom toxicity.