Seafood is an important source of bioavailable micronutrients supporting human health, yet it is unclear how micronutrient production has changed in the past, or how climate change will influence its availability. Here, combining reconstructed fisheries databases and predictive models, we assess nutrient availability from fisheries and mariculture in the past, and project their futures under climate change. Since the 1990s, availabilities of iron, calcium and omega-3 from seafood for direct human consumption has increased but stagnated for protein. Under climate change, nutrient availability is projected to decrease disproportionately in tropical low-income countries that are already highly dependent on seafood-derived nutrients. At 4 oC of warming, nutrient availability is projected to decline by ~30% by 2100 in low income countries, while at 1.5 - 2.0 oC warming, decreases are projected to be ~10%. We demonstrate the importance of effective mitigation to support nutritional security of vulnerable nations and global health equity.