Copper (Cu) is a transition metal with multi-oxidation states, and though it plays numerous roles in vital physiological and biochemical pathways including both photosynthesis and respiration, it can potentially be toxic at high concentrations. Coastal waters receive significant Cu input from a variety of anthropogenic sources which may affect fundamental biological processes including phytoplankton growth. We investigated the responses of the coastal diatom Chaetoceros gracilis to variable Cu concentrations using a local isolate from the SW coastal Bay of Bengal. The results suggested that Cu acted as a growth-promoting factor up to concentrations of 125 nM Cu (growth-promoting range), and became inhibitory thereafter (growth-inhibiting range). The cells in the control treatments had high ratios of photoprotective to light harvesting pigments (PP:LH) and high BSi:Chl-a, both indicative of Fe stress. Within the growth-promoting range, an increase in Cu supply significantly increased Chl-a concentrations, and decreased the ratios of PP:LH and BSi:Chl-a. Interestingly, iron (Fe) supplemented cells of C. gracilis revealed similar responses. We speculate that C. gracilis may utilize Cu to enhance Fe acquisition when Fe levels are inadequate. However, in the presence of Fe, the growth response of C. gracilis to variable Cu concentrations was not significant. We infer that, under Fe sufficient conditions, the need for Cu is minimized. Our study suggests that Cu plays a significant role in the physiology of coastal diatoms beyond the simple toxicological effects often investigated.