Human exposure to toxic mercury (Hg) is dominated by the consumption of seafood1,2. Earth system models suggest that Hg in marine ecosystems is supplied by atmospheric wet and dry Hg(ii) deposition, with a three times smaller contribution from gaseous Hg(0) uptake3,4. Observations of marine Hg(ii) deposition and Hg(0) gas exchange are sparse, however5, leaving the suggested importance of Hg(ii) deposition6 ill-constrained. Here we present the first Hg stable isotope measurements of total Hg (tHg) in surface and deep Atlantic and Mediterranean seawater and use them to quantify atmospheric Hg deposition pathways. We observe overall similar tHg isotope compositions, with median Δ200Hg signatures of 0.02‰, lying in between atmospheric Hg(0) and Hg(ii) deposition end-members. We use a Δ200Hg isotope mass balance to estimate that seawater tHg can be explained by the mixing of 42% (median; interquartile range, 24–50%) atmospheric Hg(ii) gross deposition and 58% (50–76%) Hg(0) gross uptake. We measure and compile additional, global marine Hg isotope data including particulate Hg, sediments and biota and observe a latitudinal Δ200Hg gradient that indicates larger ocean Hg(0) uptake at high latitudes. Our findings suggest that global atmospheric Hg(0) uptake by the oceans is equal to Hg(ii) deposition, which has implications for our understanding of atmospheric Hg dispersal and marine ecosystem recovery.