Tuna and billfish indicates Hg and MeHg accumulation through marine food webs, yet the causes of interspecies and interoceanic variation remain unclear. This study analyzed THg, MeHg, and Hg isotopes in seven tuna and billfish species from the Pacific and Atlantic, revealing bioaccumulation pattern in top marine predators. Blue marlin (a kind of billfish) showed significantly higher THg concentrations (1824 ± 1558 ng g-1 ww) and lower MeHg/THg ratios (28.6 ± 17.3 %) compared to other species. BAFs estimated via Monte Carlo simulations revealed species-specific differences, with MeHg values 16-20 times higher than THg. Hg isotopes suggest partial Hg(0) dissolution and MeHg demethylation in surface waters. Pacific samples showed higher Δ199Hg and Δ201Hg than Atlantic, likely due to differences in light and salinity. MeHg accumulation in tuna and billfish varies with feeding depth, as blue marlin, sailfish, spearfish, and yellowfin tuna feed mainly in the epipelagic zone, while albacore, bigeye tuna, and swordfish also exploit mesopelagic sources, as shown by binary mixing models and Hg isotopes. These findings enhance understanding of Hg dynamics in oceanic apex predators and offer key support for species-specific risk management under the Minamata Convention.