The in vitro effect of cadmium on apical segments of Hypnea musciformis was examined. Over a period of 7 days, the segments were cultivated with different concentrations of cadmium, ranging from 50 to 300 μM. The samples were processed for microscopic and histochemical analysis of growth rates, content of photosynthetic pigments, and photosynthetic performance. Cadmium treatments increased cell wall thickness and the accumulation of plastoglobuli. Destruction of chloroplast internal organization was observed. Compared to controls, algae exposed to cadmium showed growth rate reduction, depigmentation, and blending in the lateral branches. The content of photosynthetic pigments, including chlorophyll a and phycobiliproteins, decreased after exposure to different concentrations of cadmium. These results agree with the decreased photosynthetic performance and relative electron transport rate observed after exposure of algae to cadmium. Taken together, these findings strongly indicate that cadmium negatively affects the architecture and metabolism of the carragenophyte H. musciformis, thus posing a threat to the economic vitality of this red macroalgae.