Over the past decades, the concern about lead pollution in marine environments has increased due to its remarkable toxicity, even at low concentrations. Lead is one of the significant contaminants arising from human activities in Antarctica. However, its effects on polar photosynthetic organisms are poorly known. This work aims to evaluate the effects of two different environmental concentrations of lead (10 μg/L and 50 μg/L) on pigment content, antioxidant enzyme activities (catalase, superoxide dismutase, ascorbate peroxidase and glutathione–S–transferase), metabolome, thalli morphology and cell ultrastructure of the red seaweed Iridaea cordata (Turner) Bory from Terra Nova Bay (Ross Sea, Antarctica). The results highlighted that lead exposure decreased phycocyanin and phycoerythrin content, starting from 10 μg/L, while induced carotenoid accumulation at 50 μg/L. Catalase, ascorbate peroxidase, and superoxide dismutase activities generally increased after lead exposure and distinct biochemical features were identified in the control and treatment groups. Further lead–related effects on cell ultrastructure comprised floridean starch accumulation and plastoglobuli formation. Overall, our results suggested that the enhanced formation of reactive oxygen species in response to lead altered the photosynthetic pigment pattern, antioxidant defenses, metabolome and ultrastructure of I. cordata.