The agarophyte macroalgae Gracilaria domingensis (Kützing) Sonder ex Dickie is widely distributed along the Brazilian coast. While this species produces agarana, it is more important in the human diet. Therefore, the present study aimed to evaluate the biological effects of cadmium on its morphology and cellular organization. To accomplish this, the effects of cadmium in apical segments of G. domingensis were examined in vitro. Over a period of 16 days, the segments were cultivated and exposed to photosynthetically active radiation (PAR) at 80 μmol photons m(-2) s(-1), with cadmium treatments in doses of 100, 200 and 300 μM. The samples were processed for light, transmission and scanning electron microscopy. Histochemical analyses included Toluidine Blue for acidic polysaccharides, Coomassie Brilliant Blue for total protein, and Periodic Acidic Schiff for neutral polysaccharides. In all cadmium treatments, cytochemical analysis showed 1) metachromatic granulation in vacuole and lenticular thickness of the cell wall, 2) a higher concentration of cytoplasmic organelles, and 3) an increase in the number of floridean starch grains. Cadmium also caused changes in the ultrastructure of cortical and subcortical cells, including increased cell wall thickness and vacuole volume, as well as the destruction of chloroplast internal organization and increased number of plastoglobuli. In addition, treated plants showed a gradual increase in surface roughness, apparently the result of cadmium absorption. Taken together, these findings strongly suggested that cadmium negatively affects the agarophyte G. domingensis, posing a threat to the vitality of this plant species as a supplement in the human diet.