Diarrhetic Shellfish Poisoning (DSP) is a globally significant human health syndrome most commonly caused by dinoflagellates of the genus Dinophysis. While ecosystem studies suggest that blooms of this mixotrophic dinoflagellate can be promoted by excessive nitrogen (N) loading, it is unclear whether these effects are direct (nutrient stimulation of Dinophysis) or indirect (nutrient stimulation of prey) since this alga is mixotrophic and culture studies investigating the effects of nutrients on Dinophysis have not been performed. We established an isolate of Dinophysis acuminata from New York waters and conducted controlled culture experiments to assess the effects of nutrients on the growth of this dinoflagellate with and without its prey, Mesodinium rubrum. Dinophysis was found to rapidly assimilate 15N‐labeled ammonium and urea, AND to a far lesser extent nitrate. Cultures grown with and without prey generally grew faster with ammonium, glutamine, or organic matter from sewage effluent added than respective controls, while nitrate grown cultures yielded significantly more rapid growth only when fed copious amounts of Mesodinium. Growth rates of Dinophysis also increased with the amount of Mesodinium available in cultures and achieved maximal growth rates (0.36 ± 0.01 d−1) when grown with high levels of Mesodinium and the amino acid, glutamine. Collectively, this study demonstrates that inorganic and organic N can directly promote the growth of Dinophysis and supports the hypothesis that accelerated N loading in coastal ecosystems can promote DSP producing blooms of this species and, thus, may be partly responsible for their recent expansion across the North America.