The present work aimed to evaluate the influence of dietary inorganic mercury or methylmercury, with or without selenate co-exposure, in order to examine dietary selenium's role in counteracting mercury species toxicity as well as the effect of the Se : Hg molar ratio in the diet and tissues of exposed crabs (Carcinus maenas). From the dissection processes, it was possible to obtain for each crab muscle from legs and thorax, hepatopancreas, gills, stomach, hindgut, heart, testis/ovary and carapace. All samples were analyzed by CV-AFS for total mercury and HR-ICP-MS for total selenium. For the crabs exposed to inorganic mercury, the accumulation was statistically confirmed (p-value <0.05 by comparison of tissues from the exposed crabs to those from control crabs) in the hepatopancreas, muscle from thorax, gill and heart, whereas for the co-exposure to selenium, the accumulation of mercury was significant only in the heart. In contrast, when considering the methylmercury exposure, the Hg accumulation was statistically significant in the muscle from thorax and legs, hepatopancreas, gill, heart and testis/ovary. However, for the co-exposure to selenium, the Hg accumulation was only significant in the muscle from thorax, hepatopancreas and gills. Therefore, the accumulation of Hg and the Hg-Se interactions in the hepatopancreas, heart and muscle of the crabs were highly significant for the exposure to Hg species without co-exposure to Se. Thus, the dietary exposure to Se played a very important role concerning the uptake of the Hg species investigated in the present work, showing that it influences the uptake and accumulation patterns.