Dimethylsulfoniopropionate (DMSP) is a ternary sulfonium compound produced in large quantities by many marine phytoplankton. Although DMSP certainly functions as a compatible solute, and thus a regulator of osmotic pressure, additional physiological roles are still in question. This study offers evidence of an antioxidant role for DMSP in response to both chronic and acute cupric ion-induced oxidative stress in five species of marine phytoplankton. Intracellular levels of DMSP varied by species, with Emiliana huxleyi and Isochrysis galbana having among the highest levels. In comparison to a known antioxidant, ascorbic acid (AA), DMSP levels did not vary, generally staying around 4:1 AA:DMSP, although this varied by species. This study also provides some of the first reported intracellular concentrations of ascorbic acid in marine phytoplankton.