Although fish is an important source of nutrients, including some of the healthiest proteins, long-chain fatty acids, and essential selenium, species at the top of the food chain frequently contain large amounts of toxic mercury (Hg). The provisional tolerable weekly intake (PTWI) of Hg from fish consumption is calculated from the total concentration of Hg and assuming that all Hg is speciated as organic methylmercury (MeHg). Using high energy-resolution X-ray absorption near-edge structure (HR-XANES) spectroscopy, we show that blue marlin (Makaira sp.), a common top predator consumed by humans, contains high concentrations of inorganic Hg(II) complexed as 57 ± 10% Hg-tetraselenolate [Hg(Sec)4] and 43 ± 10% tiemannite (HgSe). The stable Hg–Se chemical bond likely attenuates the bioavailability of Hg and counteracts some of its health hazards to consumers. Thus, monitoring the concentration of MeHg, rather than total Hg, in top predators such as marlin would provide a more robust measure of potential Hg exposure and may be sufficient for food safety controls. The bonding of Hg atoms to four selenocysteine (Sec) residues in the Hg(Sec)4 complex severely depletes the stock of bioavailable Se, and quantification shows that blue marlin is not a chief source of dietary Se essential to selenoenzyme synthesis and activity.